Pardon my rants and raving, but too many times "accepted facts" in the rail fan/enthusiast/historian community are incorrect. (Unless it is more valuable to criticize a poster’s spelling and grammar misteaks.)


On that "other site", it was reported that GMTX #499 has been rebuilt into a GP15-2 with dynamic brakes. In the discussion, no mention was ever made that besides the 25 C&O/CSXT GP15T units built with dynamics, there are in fact four more GP15s built new with dynamic brakes in 1982 - for Venezuela. [www.rrpicturearchives.net] [emdexport.railfan.net]

Apparently all GP15 models were built with Dash 2 electrical cabinets and modules - but only some GP15 units are typical Dash 2. The Dash 1 versions do have three Dash 2 modules - a VR module controls the auxiliary generator; a TR module controls series-parallel/parallel transition, and an EC1 module controls main generator excitation. The EC1 module appears to be unique to the GP15 (at least for domestic models). Depending on the throttle position, it provides a basic signal, modified by the load regulator, that is output in a pulsed to a DC SCR assembly that provides a chopped output to excite the main generator.

The full Dash 2 versions are very different and have many more modules, especially traction alternator excitation which is split among several modules. There is no need for a TR module since these are in permanent series-parallel, making the VR the only common module between the two versions, though the actual VR module version varies depending on if a DC or AC aux gen is used. The excitation system contols a system of SCRs using the three-phase AC output of the D14 (common on EMD static excitation units), very different from the DC system used by the GP15-1 and EC1.

One thing I have noticed and had to deal with is that the GP15-1 uses a primitive wheel slip system only using bridge relays and through cable relays that simply drop the load and sand (if the TDS works). (Presumably a WCR system was an option.) The end result is that a GP15-1 operating under heavy load or in poor adhesion conditions will quickly stall out due to wheel slip, especially if it sister units have governor or excitation issues. Makes me wonder how slippery an SW1000/SW1500 would be since they have a similar wheel slip arrangement.

I would like to see how the Dash 2 version compares under drag rating, since these should have the WS10 wheel slip module which is the equivelent of IDAC. I am assuming that the Dash 2 units are equipped with AR-10 traction alternators, while the -1 versions use the D32 main generator. To add to the confusion, the EMD parts list shows a GP15-2 while the operating manual is for a GP15(AC). Other than looking inside the module cabinet, or looking at the D32 or AR-10, I'm not sure there is any obvious spotting feature to easily differentiate them.


Early EMD switchers were designated SC or NC for their cast frames, replaced by the SW/NW series indicating "welded" frames. The SW1000/1500 series do have welded frames made up from slabs, plates and shapes. But the interim models from the SW1/NW2 through SW600/900/1200 have "composite" frames. For 567B engine and earlier, the coupler pockets are cast; typically 567C afterwards are fabricated. The backbone is steel slab/plate as are the end plates, but the side sills, walkway and center sills appear to be one piece casting for each side. Not sure about variants or the SW1001.

On the subject of EMD switchers, older models had full series to series-parallel transition, but later units are only series-parallel. As speed slowed under power, EMD switchers do not automatically transition back from series-parallel to series requiring the engineer to take action when speed slows – either momentarily closing the throttle or changing the switch to series forestalling. Something not readily apparent (unless you look in the electrical cabinet) is if the locomotive is equipped for "high speed" operation. It was an option for EMD switchers (including the MP15) to be equipped with one or two steps of traction motor field shunting to allow full power operation at higher speeds.


The "tunnel motor" design did not originate with the SP SD45T-2, but years earlier with the EMD MRS-1 and is actually very common as standard for various models in EMD's export line. The second GP49 specification shows the tunnel motor radiator arrangement as an option - I wonder who was interested enough for EMD to show it as an option?


It is commonly stated that on GE gas-turbines built for UP, that the auxiliary engine was only used for hostling the locomotive and starting the turbine (air or main generator?). At least on the last 8500/10,000 hp version, the auxiliary engine in the A unit could operate without the turbine in the B unit, but the turbine could not operate unless the auxiliary engine was running. The turbine powered four main generators, which were actually two tandem generators side-by-side running off a reduction gearbox. The turbine did not have any auxiliary support systems. The auxiliary engine powered two air compressors (for brakes and possibly turbine fuel atomization), its main generator apparently functioned as an auxiliary generator (the turbine Bunker C fuel heating system, including in the tender, was electric), and the radiator area in A unit was actually used half for cooling the auxiliary engine, and the other half was an oil cooler for the turbine, cooled by a common fan driven by the auxiliary engine.

And while on the subject of GE's - the White Pass and Yukon shovel noses are a unique model, but not a unique design. The shovel nose arrangement was actually very common GE design built in a wide variety of variations (gauge, wheel arrangement and prime movers) before the WP&Y order. And the "Universal" line did not start with the U25B - GE had already advertised their Universal line to the international market in 1956 and was delivering units of these designs. The first of the Universal line built for use in North America were the UD18B demonstrators (part of the 1956 catalog) that went with eight more to NdeM. The first US Universal type designs built were the two narrow gauge units for US Gypsum at Plaster City, also in 1956. These predate the XP24 (U25B prototype/test units) and U25B demonstrators by several years.