The Montezuma Yardmaster Wrote:
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> I have never heard of "alignment" couplers.
> Please tell me what is different about them and
> why they are required.

Most newer units have what are called Alignment Control Couplers, which is a somewhat misleading term since it actually involves the drawbar and not the coupler. ACC prevents the drawbar from slewing all the way over when in buff. In situations involving movements through switches and crossovers or around tight curves, units (more likely, two or more units) without ACC can derail.

Southern Pacific's SW1500s are a good example. Units 2450-2679 were built without ACC, 2680-2689 (and all the MPs) were. SP addressed the problem by installing removeable drawbar blocks on units without ACC; the blocks were supposed to be in place when the units were used a road consist, and removed when used in switching service. There were also restrictions on where the units could ride in road locomotive consists.

Union Pacific has recently (within the last year or two) issued instructions that any "foreign line" unit without ACC can only be moved by itself, towed by another unit. Thus, a unit without ACC can no longer be moved in a freight train, only as a special movement. That drives up the cost.