Ok. Just picked up the Rail Crew earlier this week. The minis are great, and the plastic is a cinch to work with! I tried out the gravel quarry base with Sue, and decided it looked like rubbish. I think I'm just going to fall back on my greenstuff cobblestone standby.
Also, I can't remember where I read this, but someone pointed out that Mei looks more like she's kicking through a wooden ceiling, rather than busting out through a wooden floor head-first. I can't un-see it now, and that basically renders my conundrum moot.
Will update with pics once I've got the basing done. Thanks again for the advice everybody!