I've been wondering why most real intermodal trains use 4 axle engines while regular freight trains are usually led by 6 axle units. I'm sure there is a logical explanation for this.. are intermodal trains that much lighter than regular freight trains?
Tom, railroads assign a much higher ratio of horsepower per ton to intermoidal trains than to "bulk" freight trains. This is done to make sure they get over the road faster and aren't slowed down by grades or having to take siding for opposing traffic. The extra tractive effort provided by the traction motors on axles five and six just isn't necessary. Some railroads, like CSXT, now assign six axle power to intermodal as well, but 4400 HP and above units.
A few years ago this was a lot more common. The former industry-intermodal leader, the Santa Fe, was a big believer that four-axle units accelerate quicker as more tractive effort was applied to fewer axles. That is a big reason why the Santa Fe ordered a fair amount of four-axle units in the early 90's for use on their fleet of piggyback Q-trains and numbered UPS shooters. I know for a fact the engineers as a whole hated the four-axle safety cab units, ie, the B40-8W and GP60M, as the extra weight of the cab made it one hell of a rough ride. This policy of using four-axle units over six-axles on IM-trains was eventually more or less scrapped as the postive time differences related to fuel comsumption and overall locomotive utilization were found to be not that meaningful. Now, take a look at the ATSF Transcon CH-LA line, almost all the Pig trains have C44-9W six-axle units.
