Sir_Prize,
I must confess, that even though I do own an air brush, I rarely use it because of all the time it takes to assemble, disassemble, clean, reassemble, disassemble, clean... Whew!
Also, the engines, rolling stock, & structures I usually paint are models of an urban branch line, & sharp, crisp paint schemes are not the order of the day. I actually think a brush does a better job representing a heavily weathered piece of equipment.
For the model you mentioned though, I don't think you'll want to go for the "rust bucket" look. That's why I think the air brush would be better for your purposes.
BTW...a single action airbrush is much easier to deal with, & is really all you'll ever need for model RR purposes.
And Mike has a VERY good point...If you're a novice at painting, ALWAYS get some practice on somre kind of scrap material...old diesel shells, etc...
Good Luck!