The subway wagon is one of the first models made for advtrains. It is loosely based on a european metro/U-Bahn train, like one from Berlin or another European city. The subway train is probably the most ubiqitious train in Minetest, used on most servers with Advtrains for public transport.
There are a few derivatives of the subway wagon in different colours, but this will soon be deprecated in favour of an official method of painting (see here).
If the subway wagon has a line number property set, and that number is between 1 and 10, it will apply a specific texture to both ends of the wagon to indicate its line number. If that number is between 11 and 99, code will generate the number and a colour background to go with it.
If the line number is “X”, it will display a big red X, which is usually to indicate the train is out of service. The subway wagon was the first mod to have line display capabilities like this.