OK, then. Looks like I'm out of luck with this approach.
I just tried using the line drawing tool to draw a new line, tracing over an existng rail line to highlight it with a new, broader colored line. That sort of works, but it's kind of tedious - especially along curves, which I had to approximate with a series of short, straight lines. Also, when I get near a road, the drawing tool wants to latch onto the road instead of the rail line.
I also have Google Earth installed on my computer (free version 6.0); is there anything there that would work better for this? The thing is though, I want the resulting custom map to be viewable on-line by anyone with a browser and access to Google Maps (without them having to download and install Google Earth).