Preparation
I decided to prepare two small sections to test track weathering. First I had to add missing ties:
I then protected the surrounding tracks:
Ties / sleepers
I then airbrushed with Testors Model Master Railroad Tie Brown Flat 4885:
I immediately passed a scrap of plywood on top of the rail to remove the excess paint:
Here's a close up:
Rust on the track sides
I made a mix of Microlux Rail Brown #29001 and Rust #29005:
The paint is applied with a small brush; it should cover the sides as well as the plates:
Looks good!