Railways are a source if supplies and assets for militaries. Additionally the TrainX class could be used for weapon systems that move around on rails, UAVs launched from angled rails on a trailer, boat launches from land and on large ships that use angled rails, ect...

Railways would be more centered on user made maps that feature railways.

are there railways on altis?

No, but that is not the point.
The point is for modding, if someone wants to mod a railway on Altis, then he/she could mod in a train as well. This is more centered towards user made maps and content. Currently, the few trains we have seen in Arma games are horribly scripted nightmares. Modders could also find many uses for a TrainX class outside of the standard idea of a train.

what else could it be used for?

You could use to create a "vehicle on rails" that will drive or fly on a set of rails. The player could operate a gun and shoot at targets.

You could create vehicle shooting ranges were a player ride in the gunner seat.

You could use the TrainX class for transporting aircraft around on large ships like aircraft carriers without using towing vehicles and instead using towing vehicles built into the deck.