As far as I know, there are two types of track gauge predominantly used: 1435mm (standard or 'railway' gauge), and 1067mm (or 1069, I can't remember the last number, but it doesn't matter), which is used in subways like the Tokyo Metro. Obviously, trains built for one track gauge can't operate on a different track gauge. However, I experimented with using 1067mm trains on 1435mm routes - and the trains operate properly. Why is this?

Well, if that shocks you, try putting an EMU on a non-electrified route.

This is simply not something openBVE (or BVE for that matter) knows about. You can specify the gauge in the route file, but even that only affects how the train handles. The trains don't have a specified gauge at all - you can certainly place the wheels at a certain distance from each other when you build an exterior, but that only affects how the train looks, not how it handles or on what track it will run. Whether the trucks (bogies) actually fit on the track or not is irrelevant in the simulation.

Out of curiosity then, what are the handling differences between the two gauges?

I'm not sure about the details, but broadly speaking, a smaller gauge means less stability in curves. Some developers have actually used an unrealistically low value for the gauge as a quick and dirty method of simulating a tilting train.

Cool. Thanks.

Japanese 1,067 mm track gauge is also known as Narrow gauge. These railways usually cost less to build because they are usually lighter in construction, using smaller cars and locomotives (smaller loading gauge) as well as smaller bridges, smaller tunnels (smaller structure gauge) and tighter curves.

Narrow gauge is thus often used in mountainous terrain like Japan, where the savings in civil engineering work can be substantial. It is also used in sparsely populated areas where the potential demand is too low for broader gauge railways to be economically viable. Recent track improvements in Japan have allowed trains on 1,067 mm tracks to run at max speed of 160 km/h (99 mph), e.g. Odakyu Romance Car.

that's interesting. i can understand why using 1.067mm in mountainous areas could really add up to huge savings in amount of rock removed alone. Thanks for the info. =)

The 1067 mm track is known in the Netherlands as "Cape railways", because they were (and are) still used for railways in South Africa. In the early years of the 20th Century they were widely used for local tramways in the Netherlands.

The MTR in HK is using 1432mm for some of its railways. Then why is that?

The 1432mm might be keep to tighter tolerances (less squealing/hunting) without having to change the rolling-stock. Russian did a similar thing (1520mm track with existing 1524mm rolling stock).