My "7,500 foot" answer echoes the posts above...
CPR's customers have told CP they expect:
1. Damage free shipments.
2. On time delivery.
"Damage free" is acheived by reducing the number of times each car is switched enroute and a LARGE emphasis on controlling (reducing/eliminating) in-train forces by CP's Locomotive Engineers. A train is "designed" to go from A to Z with very limited re-blocking/yarding. This reduces the chance of a "hard" coupling. CP's yards can barely handle 7,500 foot trains, and mostly by "doubling over". Their yards are simply "small" because CP doesn't "like" yards; they build trains once and let 'em fly.
"On time" also reflects CP's business model. This doesn't mean that train #292 will ALWAYS depart Alyth daily at 19:00, but 292 will ALWAYS make it's trip from A to Z in 56 hours 40 minutes.
CP has always made their money by hauling the most lucrative shipments they could handle, which rank:
1. Heavy bulk commodities in unit trains (grain, coal, potash, molten sulphur, etc).
2. Manifest "mixed" freight.
3. Intermodal, which is dead last.
In order to make any money on intermodal, you've got to move ALOT of it. And moving alot of it would entail longer trains, a la CN...
CN has the best low-grade route across Canada, whereas CP twists and turns and climbs and drops all the way across the continent. In addition, CP's main lines are single track with sidings. CP's infrastructure simply doesn't allow for 12,000 foot trains, as correctly pointed out their "standard" siding is 8,000 feet. However, based on CP's business model, ANY unit/manifest train can meet it's counterpart using an 8,000 foot siding. To invest money in adding 4-5 thousand feet to each siding to handle two or four 12,000 intermodal trains isn't worth the expense. Trying to force a couple of 12,000 intermodal trains down the current main line infrastructure wouldn't work. It would limit all siding meets to 2 trains instead of a 3 train meet, holding back another train, and so on, creating a "damage path" of delays to the "money makers" to handle 1 low-income 12,000 foot intermodal. CP simply isn't interested in that traffic.
CPR's CEO, Fred Green, has made it PRIORITY #1 to make CP a "fluid" railroad, reflecting what their customers want and pay for. CP has focused their investment dollars in locomotives and track structure, ensuring the trains move on time. Delays and congestion have rapidly been eliminated over the last 6 months, and actually reduced the number of employees needed to move the trains, resulting in layoffs of newer employees. CP's main lines are "over capacity" based on mathematical formulas for what each line "should" effectively handle in a 24 hour period. So, CP "should" be able to handle 16 trains/per day without delay/congestion on the "Alpha" Subdivision, 31 trains/per day on the "Bravo" Subdivision, etc. "Alpha" is currently running 19 and "Bravo" is running 38; and that's without any delays...so, Mr. Green's plan/business model is working. CP isn't going to throw in a 10,900 foot IM dog and mess it up...