by v8interceptor
drgw-sd45 wrote:There is something to remember with all of this:There are still plenty of applications for 4 axle power other than high speed mainline service, this is why the big railroads are spending so much on rebuilding/remanufacturing/repowering older EMD GP's...While 4 Axles may be a thing of the past for road power, note that GE is aggressively marketing their new 4 motor (A1A trucked) version of the ES44AC....
First is that since the mid 60's four axle units have always been models identical to their six-axle counter parts. (Example GP40 and SD40, U30B and U30C).
Second is that as the horse power get's higher and higher, one has to wonder if you can really distribute 5,000 or 6,000 hp over only 4 axles. Or if you can do you get the correct tractive effort and not wheel slip.
It's interesting that back in the 70's the Milwaukee Road did a study into 4 and 6 axle units. They concluded that once a train got up to speed there was little to no difference between 4 and 6 axles. The SD40-2's were to be the last 6 axle unit the railroad was going to buy. Simply because they came to conclusion that the extra cost and maintenance associated with 6 axles wasn't worth the cost, both initial and overhead. Now one has to remember the Milwaukee had most of it's trackage in areas that were covered with high grades, also tonnages of freight cars has gone up over the last 30 years and trains have gotten longer, so who knows if that study would still hold water today. My guess is that it wouldn't, based on the fact that ATSF GP60's have been the last 4 axle units bought in North America.
Somone else on this post is correct also. There are loads of left over 4 axel units all over the place. As far as what is cheaper, rebuildilng an old loco for transfer/yard service, or buy a multi-million dollar new one? The answer is clear. Plus, many roads have started using 6 axle units in these roles. UP uses rebuilt SD40-2's for hump and yard service in North Plate. BNSF regularly uses rebuilt SD40's with remote control in Galesburg for their yard service.
In many ways, except for passenger service, I think we have seen the end of the usefullness of the 4 axle locomotive. Who knows, maybe sometime in the future this will change, but it is the trend for the forseeable future.