I think that’s a bit of a grey area, material packs are usually things the devs make a lot of money on (see: Cities in Motion 2). They might want a slice of the pie on those. And I’m not sure if I’ve ever heard of someone asking payment for UI/Gameplay mods. That’d be a new thing.
If anything though, you could do some mods you normally wouldn’t on commission and then release the end result as freeware (after payment). Plus, asking people to pay for your mod [or at least every person interested] is going to detract anyone that might be interested in doing mods for free.
Regardless, I’m glad you’re interested in my 2 trains! Feel free to mail me (not sure if the forums have a PM system) to iron out payment details and other stuff. That way, this thread won’t be needlessly sidetracked any further.
My e-mail should be on my profile page.
Lastly: The price on the FLIRT is a bit steep though; 4,2 mio for it and 4 mio for the TFV? It’s a bit… steep, me thinks.