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This page came about due to a challenge issued on RTL. I asked for a fire glossary and promised that if one was forthcoming, I'd do the same for trains. Jeff put up a good one!!!



B Unit
A locomotive without a cab. Never a Steam locomotive, and almost never an electric. Sometimes called a Booster Unit. More common among first generation units, and more common among cowl units than hood units. Railroad financial officers specified B units to save money on the cost of the cab. Railroad operating departments prefer the flexibility of being able to operate any unit in the lead, and studies have shown that the initial capital savings do not outweigh the operational efficiency savings.

Bridge Rail
Train derailments are always bad news. However, one of the worst places for a derailment to happen is a bridge. Not only would the potential loss of life and money due to the lost or destroyed cars be worse (because they fall off or strike girders, etc), but the derailment will be harder to clean up, and the bridge itself might be damaged, and the expense and effort to repair it might be enormous, and the cost of having the line out of service for weeks or months might be into the hundreds of millions.

Therefore, real railroads take extra steps (not cost effective everywhere) to defend against derailments on bridges (and in tunnels, as well, sometimes) As a train approaches a bridge, there appears another set of rails placed inside the normal rails. Usually these are tapered toward the center of the track at each approach. These inner rails, called bridge rails mean that if a truck wheel comes off, it won't go very far. In fact, with proper tolerances, the truck wheel may not come off at all. They typically start far enough in front of the bridge that there is time for a wheel to right itself if it hasn't gone too far. Model bridges simulate this.

Other steps taken before large expensive bridges can include hotbox detectors, dragging equipment detectors, axle counters and slow orders. These steps failed in the Amtrak tragedy of a few years ago where a (deck girder) bridge segment had been severely dislodged, (or completely removed, I can't remember) because it was struck by an errant barge that had strayed into the wrong channel. The defense against THAT is a continuity detector, which I believe was discovered to have been malfunctioning.

1. [Mathematics] A curve of the kind formed by a uniform chain hanging freely from two points not in the same vertical line.

2. [Electric Railroading] A kind of overhead wire, used by (real) electric trains to pick up power. It is typically used in high speed service. It is so called because there are two wires. The upper, heavier wire, carries the load, and has hangers welded (or otherwise connected, not very common) to it every so often. The lower wire, carried by the hangers (it's welded to them as well) is straight, and is the surface that the pantograph or trolley pole contacts. Both wires are electrified, typically.

It's called catenary because the upper wire describes a catenary curve (see sense 1) typical of wire or cable under load. Suspension bridge main cables also follow catenary curves. As I said, the lower wire is as straight as possible and thus is uniform in height when viewed from the side.

Done correctly, catenary can support speeds well into 200 MPH. (If everything else is right too. The track has to be PERFECT, and all curves need lots of transition both horizontally and vertically.)

Contrast with single wire overhead, in which the load wire and power wire are one and the same. This wire sags from one pole to the next, so a pantograph or trolley pole will go up and down a bit as the electric car or train moves. This is the main reason trolley poles and pantographs are sprung, but this bobbing is not suitable for high speed service.

The area of a locomotive where the Engineer and Fireman work. Cabs may be located anywhere on the locomotive.

Steam engines typically locate them at the rear, behind the boiler. Coal hand fired locomotives need to allow the fireman access to the coal on the tender, and coal stoker fired locomotives do not want a long screw run for reliability reasons. Oil fired locomotives were introduced early this century and a few steam locomotives, notably articulateds operated by the Southern Pacific, had the cab in front. This was for better ventilation in long tunnels. These locomotives were so rare they were called "cab forwards". Steam locomotives are very directional and are hardly ever operated in reverse for long periods.

A diesel is much less directionally and mechanically constrained and the cab can be located where most convenient. Cowl units usually have the cab near the front. Early road switchers had high short hoods, and a few roads (notably Norfolk and Western) ran the units long hood first, but most roads ran short hood first. Most second and third generation hood units have a low short hood for better visibility from the cab, and are usually operated short hood first.

Cowl Unit
A streamlined diesel unit characterised by a full width body from front to rear. Most of the first significant diesels from the FT on were cowl units. Often used in passenger service after hood units became widely available. Mechanicals are accessed from within the body. Early cowl units are somewhat "unibody" like in that the body carries some of the structural load, and were sometimes nicknamed "covered wagons". Later cowl units are built on the same frames as the freight service hood units whose mechanicals they share, and thus no longer use the body for structural strength.

UK term for the person who operates a (freight road) locomotive. See Engineer.

US term for the person who operates a (freight road) locomotive. See Driver.

The UK equivalent of a cab. Usually are open to the air and have no seats.

The special track component where the two inner rails of a switch cross. Located some distance from the points (the exact distance is one way to measure the curvature of a switch) this is the last place where the two tracks have anything in common. In real railroads, frogs can be cast or built up out of welded stock, but typically are made from the rails of the switch.

Hood Unit
A non streamlined diesel unit characterised by a narrow body with outside walkways, called a hood. Mechanicals are accessed by opening doors in the hood. The hood provides no structural strength, so the frame carries all of the load. Hood Units were introduced for freight service, although some were seen in passenger service. Some modern hood units have full width ("safety" or "comfort") cabs and short hoods, sharing components with similar era cowl units, but narrow long hoods.

US term for the person who operates an (electric road) power car or locomotive. See Driver and Engineer.

A device to pick up current (a Power Pickup) from an overhead wire or Catenary, characterized by a wide rubbing surface.

Pantographs have one degree of freedom. They can move up or down but not side to side, they are rigid to the rolling stock in that axis. Thus they are heavily sprung to ensure they have good solid contact with the wire. Further, to prevent wear of one spot on the pantograph shoe the overhead wire on sections of line where most equipment is pantograph equipped actually is strung so that the contact point varies. That is, it zig zags a bit from side to side when viewed from the top. The overhead is typically catenary in this application.

Points are the ground down rails that provide the magic in a turnout, the moving part as it were. They abut either the outer straight rail (when the turnout is aligned for diverging) or the outer diverging rail (when the turnout is aligned for straight) The points rails lead to the frog, where the rails cross. In real RR practice, of course, they are custom made out of the same rails as the rest of the switch is. True for expensive model turnouts as well, but cheaper ones use stamped sheet stock. These cheaper ones give a bit of edge for a flange to catch on and in N scale since the tolerances are tighter, the flanges often climb the edge when backing and take the truck into derailment. Hence "picking at the points".

Power Pickup
The actual part in US Electric RR usage (sometimes called Traction, for a reason that is obscure) on locomotives ("motors" in traction parlance (1)) or power cars that pick up current from the overhead are called Pantographs (if they are a wide rubbing surface, such as on Metroliner) or Trolley Poles (if they are a single pole with a wheel or shoe follower for the wire).

Trolley Pole
A pole like device to pick up current (a Power Pickup from an overhead wire or Catenary, characterized by a wide rubbing surface.

Trolley poles have two degrees of freedom. They can move up or down as well as side to side. They have a small wheel or shoe follower that maintains positive contact with the wire. Light rail, where trolley pole equipped stock predominates, is typically centered exactly over the track if possible, and typically is single wire. Trolley poles have a much lower speed service ceiling, probably 70 or 80 tops, with typical speeds more like 30.

I need to add definitions for the following: also write up or provide refs for: Locomotive wheel arrangements:


Steam locomotive wheel arrangements are described by a number for each grouping. A conventional locomotive has 3 groupings. We start from the front. Wheels in front of the drivers are call the pilot wheels and form the pilot truck. Called that because they find their way for the much larger driver wheels. Wheels behind trail the drivers, hence are called the trailing truck. Some trailing trucks get quite large as they are supporing the firebox, the heaviest part of the boiler, as well as the cab.

A 4-4-0 has 4 wheels (2 axles) in a pilot truck, 2 driving axles, and no trailing truck. Wheel arrangements also get names. For example a 4-6-2 is a "pacific" and has a two axle pilot truck, 3 driving axles and a 1 axle trailing truck.

An articulated locomotive has more than one set of drivers, so for example, a 2-8-8-4 has a one axle pilot truck, two sets of 4 axle drivers, and a 2 axle trailing truck.

Tenders are much less important and are not described as fully. However the total ABSENCE of a tender is denoted by T on the end of the wheel arrangement. So our 3225 is an 0-4-0T. It has no pilot truck, no trailing truck, 2 axles of drivers, and carries its water and coal on the locomotive itself instead of on a separate tender.


Diesels (and electrics) use a completely different scheme, naturally. Here what is important is what axles are powered. A typical diesel has two trucks. Groups of powered axles are denoted by by letters. an A means one, B means 2 and so forth. A hypen means a transition to another truck or articulation point, whereas no hyphen means close proxmity (axles on the same truck)

So, the most common diesel wheel arrangement in the US is the "B-B" which means two trucks, each with two powered axles.

Also very common, especially out west where track can take more weight and higher horsepower is important, is the "C-C" which means two trucks, each with 3 powered axles.

Used a lot in passenger service was the A1A-A1A. This is a two truck unit with a center idler axle in each truck. The PA1, the most beautiful diesel ever built, is of this arrangment.

The DL 9, an diesel that can also operate in third rail electric territory (into NYC Grand central station where smoke producing engines are banned, for example) is a A1A-B. Don't ask me why.

Big red, my big red diesel, is a 1B-B1 since it has an unpowered axle on the outside of each powered pair.

The DD40AX, the largest US production diesel is thus referred to as a D-D. Some refer to the U50 as a D-D as well, but it actually is a BB-BB in that it has two trucks at each end, connected by a short bridge ( a "span bolster").

The LEGO model 4551 is a 1-B-B-1. The Austrian Federal ROCO model is a C-C. I used to think it was modeled after the Swiss Federal Railways crocodile which I believe is actually a 1-C-C-1. See Roco Swiss Croc.

The GG1, a very famous electric designed by Raymond Loewy, father of streamlining, is a 2-C-C-2

Note that what I have described is the american scheme. The european scheme is different and I don't completely understand it, but our friend the austrian croc is referred to as a Co-Co.

Arguably, when we describe models, we should give two wheel arrangements. One that describes the "model" and one that describes what we actually built. For example, a 4551 is a 1-B-B-1 but really represents a C-C. Lego does not make 3 axle or 1 axle motors so we compromise.

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