For this project I have chosen to use Railmatch Frame Dirt on the underparts. This is not my usual colour for working on underframes, but I had some available so thought I'd give it a go. The airbrush I am using in this case is an Iwata HP SB Plus, because it gives me a great deal of control over the area to be covered for each pass along the model.
When working on the whole length of a model, whether it is the chassis, bodysides or the roof, I like to have complete control over the area to be covered rather than just blasting away and then cleaning up afterwards. The two main ways of achieving this control are masking and distance from the subject. In this case I'll be carefully positioning the airbrush before pressing the trigger to ensure I only cover the wheels, brake gear and, probably, the lower edges of the frames.
I won't be doing any masking.