This is... I...
WOW.
How the hell did I miss this? I hope I get to see it in action sometime. In the meantime, allow me to gush for a moment:
I have to give props (lol pun) to the use of the car lift; I've only seen this once before, even though you'd think that's what it was made for. The truck's blue brother, however, like that Hudson-esque car that crops up from time to time in custom maps, is too aqua in broad daylight. Perhaps blue it down a little.
I don't know what a British tank engine is doing in the same map as a pickup truck with US plates, but it's a hell of a nice model anyway. But I zoomed in to get a closer look, and noticed stairstepping on its white stripes.

It's not noticeable from the distance players actually see it from in-game, but if you were to ever release the model it might be an issue.
It took me a while before I realized, holy crap, the engine is actually a custom model and not a reskin of the default train. That's dedication, right there.
Brilliant use of the Portal Catwalk Principle, here. There I was, running around, thinking "Hey I wonder if I can get up to that... nah, it's probably just playerclipped background stuff." And then, where should the map's only available route lead me but that very ramp! Also, obviously, the model rules.
As for your issues with t-junctions, I bet they're avoidable. This map doesn't really seem
too much more complex than most of Valve's; I bet they use some other method to keep their stuff under the limit. I don't mean this as a put-down of any of the custom models you used in place of brushes, because they rock, but Valve is
very conservative about using models when brushes would work just as well, in part because models can't be lightmapped.
Remember, we're a mapping community. Helping with these kinds of problems is
what we do. Besides actually making maps and resources for them of course. Don't hesitate to ask about this stuff.