Boy you are asking a question here!
(All the following paints are Folk Art)
Like you for aged concrete I use Linen 90% of the time. On occasion I have also used Light Gray and Parchment.
For weathering wheels I use a base coat of Burnt Sienna and then a sloppy coat of Asphaltum (90% of the time), or Raw Umber, Burnt Umber, or Raw Sienna (for variety).
For weathering trucks I use a base coat of Charcoal Gray and Dry brush Burnt Sienna and Asphaltum.
Weathering track and ties I use Asphaltum.
For weathering rolling stock and locos I use Charcoal Gray on all. I then use a combination of one or more of Raw Umber, Burnt Umber, Raw Sienna, Dark Gray, Licorice, Country Twill (for covered hoppers carrying cement).
To make rust spots I have used a combination of layers of Burnt Sienna, Red Oxide, Real Brown, Huckleberry, and Maple Syrup.
To paint boxcars I have used Huckleberry, or Barn Yard Red for redish cars or Maple Syrup for brownish cars.
I always recommend testing and/or practice before you use something just to make sure it is what you are looking for.