I have an idea for a mod which is to add a stage where you can pre build a train just how you want it without paying for all parts for your new train until your finished with your design and then be able to save it as a templete even.
This would allow you to design your train with the correct engine, the correct wagon and number of wagons for the train and then you can buy that whole train all at once or save it as a template for later (for example if you need to wait for money or new a second train later).

This would probably be very similar to how replacement of trains works where you can upgrade or repair your trains but instead of being for all existing trains it would be for all new trains yet to be built.

Just an idea i got while playing and I thought that i might be both convenient and cool as an addition to the game.