Describe the feature reqest
The train data saving is fairly inefficient now as each time a train is updated, all other trains need to be saved; it is especially obvious on servers with large maps as train data for chest carts in unloaded mineshafts are rewritten as well.

Describe the solution you'd like
Put train data in a folder instead of in a single railcraft.trains.dat file.

Describe alternatives you've considered
I haven't thought of many other good ways to improve performance yet.

Additional context
Thanks @Time6628 for pointing this out.

Chat log: (In @SpongePowered discord)
https://discordapp.com/channels/142425412096491520/142425521391665153/547204928821526529

Bug in #1724 is actually causing the performance regression. Closing.