Something must have happened over at YouTube because I remember your early videos where you had the rolling stock mounted camera for filming as the train moved and those were always a decent brightness. Are you using that same camera handheld to do your recent videos? If so, and going by what you say about your vids being nice & bright on your computer monitor then I'd put the onus of the problem on YouTube.
A thought just occurred to me: what gamma setting are you using on your camera and the files you're sending to YouTube? There's two common ones: 1.8 and 2.2; one of those will be darker than the others. Try changing that and see what happens when you upload a video to YT.
I know we've talked about this before, but did you ever get your high speed internet access setup? I know you were talking about going with a satellite feed. I'm thinking that may be your bottleneck. Something is throttling back your upload speed. Check your modem setup and stuff and maybe call your isp to see if they can help. I don't work on Windoze machines anymore; all Mac OSX Snow Leopard here; I can only give some general things to look at.
Still it shouldn't take 6 hours to send a 12 minutes video anywhere. What's the file size and what's your upload speed really? Not what your isp tells you it should be but what are you actually getting?
de N2MPU Jack
Modeling the NYC/NYNH&H in HO and CPRail/D&H in N
Proud NRA Life Member