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.
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.
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.
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.
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.
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.