The pitch:
Pretty much everything in the game can be automated, except for the construction of trains. For the bigger bases, people have 100+ trains, with 10+ cars each running around. Performing an action (placing a train wagon) manually over a thousand times in a game about automation... UNACCEPTABLE! :p
Two possibilities...
The plan (1):
The "Train Deployer". It would be a rather simple thing, think of it as an inserter that plonks down railcars. It sits next to the track. If provided with power and a car in the inventory, it will do the placement. If you place it opposite an LTN depot station, and place the deployers exactly 7 tiles apart, you can build an entire train and send it off into the network just like that. Fuelling should be easy from the station side of the track. The only missing link for this system would be placing the trains in auto mode, instead of manual. A pretty simple setup, and it should be easily customizable for different train lengths and compositions.
The plan (2):
The "Rail Yard". This will take some more explaining, and will be harder to do, but it would make for a much more engaging experience, and much more interesting setups. This requires a building of 6x4, standing "over" just enough rails to place the railcar. Instead of at the side of the rails, it will sit at the head of a rail line. A special little railcar would ten roll up, and pull the placed railcar out. You'll need a trainyard suitable for the little helper car to arrange the railcars into the train you want. It can do that at an LTN station, which then sends it off. Hard to program, lots of challenges and questions to figure out, but I believe it will challenge people to build some cool ranging setups.
Both plans require integration with the LTN mod. Without it, there's no feasible way to auto-assign trains to a schedule anyway, so no problems there I think.