Re: Alignment control couplers - Early engines, especially switchers, were designed to switch cars around sharper curves than are now typical. Alignment control couplers started becoming standard circa 1965(?) which allowed the couplers to stay centered when in heavy buff/shoving. Regardless of coupler type, you have to be careful shoving with more than 250,000 pounds of effort (generally assumed to be 12 axles of older full sized diesel electrics) since this is enough power to shove a flange over the top of the rail, especially in a curve or switch. The old couplers made the situation worse because they could become misaligned between units and shove them off the rail. There were always specific special instructions for handling engines (dead or running) without alignment control couplers or limiting blocks in consists. The railroads have just gotten so bad at special moves they can't seem to bother properly placing cars any more. Having Dupes on the rear just adds to the potential problems. I've seen recent UP trains in grade/curve territory that have an awful mix of loaded and empty blocks, shorter and longer cars, and too many empties close to the power for my comfort.

Alignment control couplers were not standard on any switcher I am aware of until recent years. I'm not even sure they were an advertised option. Some railroads may be retrofitting them, and recent rebuilds likely have them (at least in the west).

Most primary suspension truck designs (think type A switcher trucks) are limited to 45 mph, at least when leading, because they don't have any lateral suspension to sway into curves and track irregularities. The Flexicoil B and Blombergs have lateral suspension, but I do recall SP typically restricted the Flexicoil B to 65 mph.

Another factor is gearing - if they have low speed 65:12 gearing they are limited to 50-55 mph to keep from disintegrating the traction motors due to centrifugal force. From the best I can tell, high speed transition was always an option on EMD switchers. Anything above 25-30 mph and the cemf would start killing your tractive effort unless the engine had one or two steps of traction motor field shunting. If I recall, the SP MP15DC units have one step of field shunting, allowing them to develop closer to full tractive effort at higher speeds.

Talk about not looking about how much your train is bouncing around - I used to run a cow and calf on type A trucks at 45 mph on the BN main with welded rail. There isn't any speed restriction with the calf leading, you have 95% of the same visibility as if the cow was leading and better than long hood forward on a roadswitcher. That calf jumped and bucked and did everything but roll over - which I thought was going to be a real possibility one day when a sugar beet semi was thinking of running the crossing.