Hi zwl,
In HO the MTH steam engines have been run in batches just like in O scale. However, because they have fewer tools many of the engines have already been cataloged a second time. The only ones you might find hard to get are the Pennsey K4's and the USRA light mikados. Both of those were run once and have not been re-cataloged. The K4's were the first MTH HO engines to ship in 2006 and are pretty well gone from dealers. Other steamers like the NYC Mohawks, NYC Empire State Express, UP 4-12-2's, and N&W J's have only been cataloged once, but are still easy to find. The HO market expects things to stay in stock for a while, so the production runs are not only larger than in O, but the percentage of pre-orders is lower. Some of the real winners, like the Erie Triplex, Challengers, and SP GS-4's are on their second or even third run.
The diesels are a different story. MTH always seems to have multiple runs to satisfy demand for their diesels. They managed to be the first to bring a plastic SD70ACe/SD70M-2 to the HO market and it has been a HUGE seller. They're on the sixth or seventh run of CN SD70's. The same has largely been true for the other diesels as well. Just FYI, the PA-1's are a truly unique item in HO. They are the only diesels ever produced with smoke units. That hasn't really caught on with old timers in HO who have bad memories of slobbering seuth type smoke units, but it seems to be catching the eye of many newbies. The diesels also have functional remote controlled couplers that work best with Kaddie #58 HO scale couplers.