The engine that spends the most time on my o27 loop is a postwar 675. I also run Turbines on that loop (the Century Club version is hard to beat!). My 2046 had a problem with the switches on my last layout. I had to modify my track plan to include a small (2 or 3 inch) straight on one side of the switch, wish I could recall which one, and all was well. I also like Lionel's docksider.
For diesels, I like the SD-18s from the MPC era, or the postwar 624 switcher. The postwar 520 boxcab make for an interesting train, but it doesn't pull much. The GP 7, and all of MPC's variants (GP9, GP20, U36b, U36C, SD18) of it seem to do fine on my layout.
When I designed my layout, I tried to make it so that none of my switches used the curved leg as the main line. As a result, almost all of my running of the above engines is through the straight portion of the switches. My last layout wasn't planned like that and derailments were more common.
Hope this helps,
J White
