Back of the napkin calculation: the paint needed to cover your average freight train weighs as much as an extra full tanker car added to the end of it. amirite?

Show your work.

Wet or dry?

How long is the train though?

The graffiti adds another 4 trains worth.

You are WAY off.
An average boxcar has dimensions of about 55' long, 17' high, 10' wide.  
Area of sides = 2x55x17 = 1870 sq ft
Area of ends = 2x10x17 = 340 sq ft
Area of top = 10x55 = 550 sq ft (I assume you aren't painting the bottom)
Total area = 1870 + 340 + 550 = 2760 sq ft, or 3310 sq ft if you want to paint the bottom
 
Let's split the difference and say approximately 3000 sq ft per car.
Paint coverage for one coat = 200 sq ft/gallon, approximately,
so it takes about 3000/200 = 15 gallons of paint to do one car with one coat.
 
If the number of cars in the freight train is 100, you would need about 1500 gal.
 
Approx tank car capacity = 10,000 - 30,000 gal, MUCH more than what you would need.