I enjoyed driving the Doodlebug tonight. Thanks for your good work. I think the engine sounds are good, but like an earlier post, they could be a little higher pitched to reflect a six-cylinder diesel. Also, I talked with an old friend who worked for the Piedmont & Northern RR (South Carolina) driving electrics. He said the P&N whistles were smaller, single chime, air whistles (not air horns) that were higher pitched than most of the diesels in RW and more similar to a smaller steam whistle. Therefore, one of the european single chime steam whistles might be interesting.
One other suggestion is to mount pilots (cow catchers) on both ends of the car. Many shortlines didn't have reversing facilities and ran their doodlebugs in reverse on the return trip. If you have time, why not add a trailer for baggage and LCL loads.
Thanks for your good work.