I asked this in the BNSF forum with a bunch of other railroad specific questions, but this one is a bit more universal. Is there any Ryhme or Reason to locomotive pairing any more? Example, I saw 3 SD40-2s and a GP 50 heading a Stack train. Or Dash 9-44CW paired with a Dash 8-40BW. 10 years ago I seem to recall SF having the policy of 4 Axle units on Q trains. What prompted the Change? Are six axle units capable of higher speeds now?
I am in no way an expert on this subject, but will offer my observations that come based on discussions with engineers as well as reading other discussions: When a consist is put together, speed is not as important as adhesion. Most freight locomotives today are geared universally, meaning all have roughly the same top speed. The old standard freight gearing was 62:15 which usually gave a top rated speed of around 65 mph. Today the standard gearing may be a little different, and I don't have any numbers. The train you saw was put together not as three SD40-2's and a GP50, but as 12,500 HP with 22 axles. As for the old Santa Fe method of running high HP 4 axle units on Q/Z trains, it mainly was because these trains were usually short, relatively light (under 6000 tons) and were given more than enough power. High HP 4 axle units are "slippery" by definition, having 1000 or more HP per axle. This is not a problem at speed or with a light train. Where this becomes a problem is with low speeds on grades and with heavy trains that require more tractive effort than horsepower. As I noted, these are just my observations. Hopefully someone here with more firsthand knowlege will help fill in the blanks I didn't fill.
Modern 6 axle power is usually equipped with radial trucks, that is the axles can steer within the truck itself. This allows higher speed with less rail wear. It is still prefered to match similar units such as AC traction units with same, high adhesion DC with same and so on. Norfolk Southern recently adopted a consist matching policy whenever possible. In particular the AC traction units will not perform as well when paired with earlier DC units such as an SD40-2. The AC units, especially if in the lead, would try to make the older units work the same as they do and cause damage/overload to the older units. There is circuitry to prevent this in the newer units, but most roads will put the older DC units in the lead as a precaution, negating the advantage of newer wide cabs and technology. Everything pretty much will MU with everything else on current mainline RRs, but the efficiency of matched units can make a difference.
Trainboy, check out page 26 in the June 2002 issue of Trains magazine. The NS has begun to match power in MU configurations in an effort to reduce maintenance costs. I won't go into it here because the article does a good job of explaining the technical rationale.
