Scalecoat Model RR Paint
Scalecoat II Touch Up Paint Pen for Use on Brass, Metal & Wood

Boston & Maine Maroon
Walthers Part # 640-21233, p. 842 Walthers 2012 HO Scale Reference
A scale, $5.29, currently in stock at Walthers

 
Ideal for use on plastic. Cleanup with Scalecoat II thinner.