Fred - here's another option:
Floquil Railroad markers
FLOF3803 – Weathering markers track colors: Rail Brown, Rail Tie Brown and Rust
FLOF3803 – Weathering markers earth tones: Aged Concrete, Weathered Black, and Roof Brown
These were developed for model railroaders, not diorama builders. The rail brown and rust are particularly helpful, as you can very quickly age the sides of rail, while leaving the top shiny. Or, if you do get some on it, it's quickly removed with a track cleaner. For plastic track, you'll need something else to color the rail top; or, use the rust if it's an abandoned line. You can find these sets for about $7 at discount hobby stores or eBay.