By and large, I'd have to say no.
There are a couple reasons previous posters made:
mm00047 makes a good point. What kind of kit? How complex? People have been talking about plastic locomotive kits for as long as I've been a forum member (at least). Haven't seen anyone really try it. Heck, we haven't even seen locomotive customization or super-detailing kits really take off, have we? I think they are big in HO, but not here.
rex pointed out that a kit isn't really going to save you any money. By the time you get everything you want or need, there probably isn't any savings. To me, the only two benefits of a functioning locomotive kit are 1) you get a locomotive that is rare or 2) kit form saves you money. The complexity is also a limitation for me.
dkdkrd is right as well. Kits let manufacturers off the hook for function, fit, completeness - too much opportunity to cut corners. Unfortunately, it's not the 1960's anymore, modeling-wise. (BTW, anyone who tells you they can remember the 1960s wasn't really there.
). Replacement parts would be very difficult to get given the realities of manufacturing in China.
To those who wish to try these, my hat's off to you. But I don't think the market is big enough to support the idea.
As for kits for freight cars, I have one from Ye Olde Huff n Puff that I'm going to build and I've scratch-built my own ingot flatcars. I'm not adverse to freight car kits, but a working locomotive? I dunno...
George