Is it possible to re-use original TTD graphics under any license whatsover, or if not, under no license at all?
If you have the rights of some work, you can relicense it to anything you want.
Since you're not the owner of OpenTTD sprites (seems likely at least), you cannot change its license, or distribute it without permission of the owner (unless you can provide proof that he has given such rights away, but given you are asking, you likely don't have that either).
If you want to go ahead anyway, and use the original sprites, you should probably ask a real lawyer instead of some $random noob people at the Internet.
Copying the graphics doesn't make it your work. "Inspired" well yeah, but as a precaution I'd use a free set of tracks as source of inspiration instead. Much less dangerous. Ideally you'd never have seen the tracks, but that's a bit difficult
I think you should aim for an improved set rather than a copy, since they already exist, and it's pointless to make it again.
As for toyland being an eyesore, nah, you get used to it, really
I played hours and hours in that climate, it has awesome industries!
Alternatively, use OpenGFX+landscape (if I remember correctly), it provides temperate green in toyland climate, so you can play your favourite industries in lucious green grass.