1960 era CNW Commuter trains coasting over the diamonds at Deval Interlocking

[GN_Fan]

Back in the early 60's I lived in Arlington Hts on the CNW's Wisconsin Division.  As often as I could I would go down to Deval Interlocking just outside of Des Plains to catch the action over the diamonds.  I noticed that all of the CNW's commuter trains dropped to idle and coasted over the diamonds, even if under full throttle out of the station.  I haven't noticed this in any other location or on any other road, and was wondering why this practice was followed here and if it is still that way today under Metra.  Does anyone know?

In a second question, I also noticed that when notching up leaving a station stop, the locomotive would drop to idle and enter transition, then resume accelerating.  I know that this has been automated, but does anyone know if this is still done?  In watching CalTrain in NOCAL, I do not see this happening and love to know some of the details if the transition is still even needed.

 

[Overmod]

GN_Fan

I noticed that all of the CNW's commuter trains dropped to idle and coasted over the diamonds, even if under full throttle out of the station.

This is always a recommended practice with DC traction motors.  For a variety of reasons, in particular the induction of 'flashovers' (which are devastating both to motors and main generators), EMD and probably other builders specifically advised closing the throttle 'before' encountering known vertical shock locations such as road crossings.  As noted often at Rochelle, diamonds are particular places of high shock that is almost impossible to relieve, even temporarily, with the best of maintenance.  And the likelihood of severe electrical problem is much greater when high amperage is involved, as it is during commuter-train acceleration.

"Transition" is a feature of any locomotive with DC motors; it reflects how the individual motors are grouped to make them more effective as speed increases.  (We have had several discussions on the precise details here in the past.)

AC locomotives don't have transition because they use a different method of supplying power to the motors, which involves synthesized alternating current at 'just the right frequency' for best acceleration or efficient running.  Each motor is connected to an inverter that produces the necessary waveform for it -- either permanently connected one inverter per truck, as in older locomotives built when power electronics were much more expensive, or one inverter per motor.