Canadian Pacific Diesel Designations - Trains Magazine

The RS-18u's were called DRS-18, Diesel Road Switcher, 1800 hp.

D for diesel, (DE for diesel-electric before 1948), and H for hydraulic.

The next letter(s) was for the type, S for switcher, PA passenger-A, PB passenger B, FA freight A, FB freight B, RS roadswitcher, RF road  freight. After the dash was HP rounded down to the nearest hundred, followed by a small letter for the order number.

2816 is a H1b, all Hudsons were H class.