Why more horsepower?

[Ulrich]

Why do railroads prefer more horsepower on a single frame  verses two smaller locomotives coupled together? Two GP35s for example offer 5000 hp spread over 8 traction motors verses an AC4400CW with 4300 hp spread over six motors. The two GP35s could also be pulled apart for yard duties or light shunting and work over light track that cannot support the heavier locomotives.

I know GP35s are old..but let's assume that moderized versions were available...

[Anonymous]

More horsepower makes it sound cooler...

My guess is that there's approximetely twice as many moving parts on two units as there one. Also, if you needed 10 units per train (on a Powder River coal train with three units on the head and two on the tail) then they would stretch out pretty long. More slack, more drawbars/knuckles to break...

Just my

[AgentKid]

I suspect the cost of theoretical new GP35's wouldn't be very much cheaper than the AC4400CW, making the cost of two 2500 HP unit's much higher than a propotional cost increase from 4300 to 5000 HP.

AgentKid

 

[tree68]

There are a number of ways to approach this.  In the Diesel world, UP tried one big platform (DD's, U50, C855) and moved away from the concept.  Even the Big Boys, et al, were generally very restricted to where they ran.

There are also adhesion considerations.  The GP-40 is/was a fast locomotive (3000 hp, 4 axles), but by all accounts was also a tad slippery.  Modern controls help moderate that, but it's still a factor.

IIRC, UP's E units are now, in fact, GP38's, with just one prime mover turning the 4 powered axles.

The building block approach has been a factor since Diesels first appeared - one crew, as many individual locomotives as practicality, etc, will allow.  It's worked, and continues to work.

Being able to break up a pair of locos is simply part of that building block approach.

[CShaveRR]

Someone with more expertise than I will have to address the relative tractive efforts of old diesels vs. new.  You see a bit of unit reduction in each succeeding generation of locomotives, but I suspect that tractive effort has held fairly steady in these consists.  Keep in mind that a 130-car coal train weighs a bit more now than it used to (figure 220K GRL for the 70-ton cars that would follow GP7s, 263K for the SD40-2s, and 286K for the SD70ACEs--so about a 33% increase in gross weight for a train of "120-ton" cars, as opposed to the same number of "70-ton" cars).  You might have found five GP9s (8750 hp) hauling your 70-ton-car train, maybe four SD40-2s (12000 hp) on the train of 100-tonners, and probably three ACEs (12900 hp) on a train today.

[zardoz]

Depending on the service situation, sometimes more smaller locomotives are used, such as you mentioned about being able to break apart multiple small units to serve a customer where the siding would not support the weight of a big unit, or when the big unit will not negotiate the tight turns.

There is also the aformentioned parts considerations (more units, more parts needed).

Also not only has tractive effort increased somewhat, the electroniocs used to keep the wheels from slipping have greatly increased the effectiveness of the new units. For example, in the days of 100-car 100-ton coal trains, the CNW could, on a dry day get the coal train up West Allis hill at a whopping 2-3mph with a pair of SD40-2's; if it was raining or otherwise damp, the wheels would pick up on the multiple road crossings and you would stall.  With 130-car coal trains and two new units, they can make the hill at up to 10mph in almost any weather.

[arbfbe]

More locomotives means more employees will be needed to repair and maintain them.  If the railroads reduce the numbers of locomotives they own they can also reduce the numbers of employees they will need to hire to keep them running.

It is all about fewer numbers of employees on the railroads. 

[JayPotter]

Here's the approach that CSXT would take to the comparison between a two-unit GP35 and an AC4400CW.

CSXT generally dispatches locomotives on the basis of tonnage ratings (which are a function of tractive effort) and uses the SD40-2 as a benchmark when comparing the tractive efforts of various model locomotives.  A GP35 was rated as .614 of an SD40-2; and a class CW44AH AC4400CW is rated as 1.95 of an SD40-2.  So if a GP35 were going to produce 50,000 pounds of TE in a given situation, an SD40-2 would be expected to produce about 81,400 pounds; and an AC4400CW would be expected to produce about 158,700 pounds.  So the hypothetical two-unit 5000-hp GP35 would produce about 100,000 pounds of TE; and the single 4400-hp AC4400CW would be expected to produce about as much TE as a three-unit GP35.