Prototype: why diesel-electric over diesel-hydraulic?

[ALCOSTEAM]

The AC traction locomotive while very expensive is pretty much the machine that railroads were looking for. They have phenomenal starting tractive effort ratings and due to alot of electronics they can keep continuous T/E ratings well into the mid to upper 30% range where a modern (2nd or 3rd generation) DC locomotive might do good to have 20% at 35mph and will continue to fall as speed increases. If you look up starting T/E and continuous T/E ratings on early diesels its easy to see why railroads had issues with dieselizing. But by the mid 50's advances in electrical controls and increased horsepower along with the cost savings of reducing shop costs and locomotive shop workers by 80% or more meant steam no longer had a chance even with some of the more modern steam power having better T/E numbers at speed than new diesels coming on line.

With the AC traction there is also not much worry about short time ratings on the motors and with no carbon brushes constantly wearing down there is pretty much no maintenance on AC motors (and no flash over from carbon tracking). A DC motor with ongoing inspection and maintenace is good for 200K miles or so. On an AC traction motor IIRC correctly the teardown for inspection and IF needed an overhaul is at 2 million miles.

[Kimball McGinley]

In reading magazines and websites devoted to railroads in general; the KM locos are often credited for being very successful at one thing; they got the US manufacturers into building higher HP locos. As I understand it, EMD, GE ALCO etc. did not seem to take S.P.'s desire for 4,000 HP or so seriously, until S.P. went to Germany and bough the KM's. Then their offerings began to include the high horsepower units. U.P's turbines had a similar effect.

[WJH]

The GE44 ton I took measurements from is the one at Niles Canyon. And yes, I also took the tour of the SP KM locomotive while I visited. Last I heard, somewhere in France was a remaining hydraulic drive motor they were looking at buying. Apparently it's only an external renovation, the guts are missing. If anyone is in the know, please correct me.

[Kimball McGinley]

Regarding the KM at Niles; they plan to get 1/2 of it running. These had two diesels and two trucks, one diesel per truck. They could not distribute power from one diesel to both trucks, for example. They figure one diesel and one truck will be plenty for their purposes. The second diesel was left open to rain (by the SP) and is just a big lump of rust. Hydraulic parts and gearboxes are also very hard to come by.

[Mr Ron]

Electric traction on Eastern railways was developed so trains could enter cities like New York, Philadelphia and Washington DC where steam and diesel were prohibited. The NY, NH and Hartford RR has diesel electrics that can operate from external electric pickup in tunnels into NYC. A diesel/hydraulic wouldn't work on that service.
The KM engines proved to be outstanding movers of tonnage, hauling 7000 tons up 2% grades. DRG had 6 units and when they reached 200,000 miles, they were sold to the SP. The SP was impressed with them and ordered 15 more units. Sp was able to assess their economic and technical performance. By 1968, the hydraulic transmission could provide a reliable means of propulsion with competitive maintenance costs, but the engines suffered from "complexity of construction and inaccessibility for repairs"."Air intake problems arose in tunnels and the pneumatic controls were troublesome". When the units came due for heavy repairs, they were withdrawn from service. This was not quite the end of diesel/hydraulics on the SP. In 1964 Alco built three CC versions of it's Century series. These showed promise, but the closure of Alco in 1969 ruled out any possibility of further development.(Historical facts from Brian Hollingsworth and Arthur Cook)