@Joe Fauty posted:Question - when a locomotive with cars attached slows down in a curved section is it a lack of power drop or is it the locomotive having to work harder?
The slowdown occurs because the locomotive is working much harder due to the increase in friction. The sudden, added friction from entering an un-eased curve (i.e., uniformly curved sectional track) is much greater than the constant friction from the trailing cars. I'll bet if you run just the loco (and tender) by itself, the slowdown would be even more pronounced, to the point of being noticeable and unrealistic.
I'll also bet that the loco has at least one pair of rubber tires on opposite sides of the same axle. Bad idea. On sharp curves, the inside rail is shorter than the outside, so one wheel or the other must be allowed to skid. Unfortunately rubber tires prevent this. Because most operators don't know better, 3-rail locos are geared for fast "toy-train" top speeds. But at normal speeds, the motor(s) isn't turning enough RPM to overcome that sudden increase in friction when the loco enters a curve. So you get a slowdown. On sharp tabletop layout curves it can be pretty significant.
Instead of using an appropriate gear ratio and large flywheels (which have been used successfully for YEARS in 2-rail O and other scales), 3-rail manufacturers chose to address the problem by adding closed-loop "speed control" that instantaneously feeds the motor more voltage to overcome the added friction. They also get to charge an extra $200 for it 
. If the increase is done quickly enough, it's hardly detectable. But it can sometimes make running the train a robotic experience. If it's not done thoughtfully, you end up with a "lurch," surging, etc. Electronic speed control is a complex solution that may require an additional sensor, and introduces new failure modes. It's widely accepted now, but not strictly necessary, and not my favorite thing about our trains.
