I've compared my ST unit with my IM tunnel motors and, while the ST unit is clearly nicer and much more detailed, the differences aren't super glaring at a normal viewing distance but stand out more on close inspection. To me, the things that stand out the most are the walkways (tread vs. no tread), the side grills (mesh vs. etched and see thru vs. not see thru), the top radiator mesh pattern and details and the handrails (especially the front handrails on IM units which are notoriously warped and misaligned). There is also a seam visible down the center of the long hood roof of the ST unit that is absent on the IM units. Probably less obvious are the incorrect radiator fan access doors on most or all the IM SP SD40T-2's (bolted vs. latched), which ST got correct on at least the SP unit I purchased. However, the dynamic brake fan treatment is almost identical between units, except the fan blades are painted silver on the ST unit. The units also appear fairly close with regards to overall dimensions with the exception of at least some of the IM "snoots", which had the wrong length nose, lacked anti-climbers and I think also had some brake ratchet vs. wheel errors. I don't plan on replacing my half dozen or so IM units, but I may try to upgrade or add some more details to more closely match the ST units. I have already lengthened the nose and added an anti-climber with new front handrails on an IM SP "snoot" that is also equipped with a LokSound decoder, so it's certainly staying on the roster....
