I think the inordinate amount of fuel consumed at higher speeds was the primary factor behind the demise of trains moving more than 70 (I always heard that those “fast forties” and DD40Xs were geared for 80, not 90). The last of those “fast forties” were built in 1979, about the same year that gasoline prices were first going up over a buck a gallon. By 1981, some of these units were being geared down.
There have always been freight cars with restrictions on them, and no doubt there were plenty of cars that couldn’t have been included in UP’s hot freights. But by the time CNW was merged into UP, there were more types of cars carrying some sort of restriction below 70 than there were that could be run at 70 (which by that time was the maximum for any freight car). Any friction-bearing cars were restricted to 40 m.p.h., before they were removed from service altogether. Some restrictions were due to cars being derailment-prone at certain speeds (empty tank cars can’t go more than 50 on UP); it’s possible that ongoing truck-track-dynamics tests didn’t turn up these problems until about the same time as fuel costs went up. Other cars required constant-contact side-bearings or their maximum speed was cut down. But still others, such as empty gons and bulkhead flats, were restricted in speed because of their aerodynamic resistance, which translated into higher fuel consumption.
Jeff, I was trying to locate my old UP System timetable (the one that first included CNW, back in 1995), but the dungeon is in disarray, so I can’t use it to add to the lines on which cab signals are/were used. Probably just as well–old eyes would need a microscope to read that tiny print!
The argument for widening highways is often presented to the public as increasing speed and reducing delays, but the real argument is for more vehicles per hour through the route, even though the speed on the route still remains below the posted limit. It’s not speed, it’s logistics.
But as the speed goes up, both stopping distance safety considerations and driver behavior result in the cars spacing themselves farther apart, hence the vehicles per hour per lane on the route can actually decrease. Back in engineering school, we read a summary of a study of this for one of the highway tunnels under the Hudson River into New York CitY. As best as I can recall, the optimum speed was somewhere in the range between 30 and 40 miles per hour for maximum throughput [the study was more precise as to the figure - my memory isn’t, though]. So ‘It all depends’ on the pre-widening conditions and data, as compared with the post-widening conditions and data, as to whether any and how much net benefit is actually achieved.
In the railroad context, though, the many cars in a train are of course at a fixed and constant coupled distance from each other, regardless of the speed. So for that factor alone, faster ==> more throughput, in direct proportion. However, there also needs to be a safe stopping distance spacing between trains, the same as with highway vehicles. The difference is that with advanced signaling covering several blocks ahead, the trains can still run on pretty close headways, not much longer even with a significant speed-up - I’d guess at around 2 to 5 miles for Class I freight trains, ‘depending’, on local conditions, so not much of the capacity increase wold be lost from that - porvide that there isn’t interferecne from other slower trains on the same line, of course [ and, 'If pigs had wings . . . ’ [:-^]]. .
UP has lots of ACS. Removing it would require FRA permission. FRA isn’t very willing to approve signal “downgrades” (even if the “downgraded” signalling is consistent with what most other railroads do under similar circumstances) unless there has been a major change in traffic. If UP ever operated 90 mph freights, I doubt this is the reason they were slowed down. It’s more likely because, as suggested by several of the other postings, a 90 mph freight doesn’t get you very much. You can squeeze more time out of terminal handling than you can by operating this fast, with all of the headaches that go with it.
Track elevation in curves was reduced to allow for slower speeds . Trains restricted to slower speeds ( 40 mph or lower ) created dangerous conditions , Slow speeds or trains stopped in curves with elevation ( I remember elevation of 9 inches ) were dangerous .
The Union Pacific Coded Cab Signal System extends for a few miles around The Dalles, Oregon. Apparently they get a lot of fog through the Columbia River narrows, so the cab signals really are an important adjunct for keeping the trains moving.
Additional Coded Cab Signal installations include
Council Bluffs-Ogden-Salt Lake City,
South Morrill, Nebr. to O’Fallons, Nebr. where the Powder River Basin funnel connects with The Overland Route mainline,
Gibbon, Nebr. (on The Overland Route mainline) to Marysville, Kans., and
portion of the old Saint Joseph Branch from Hiawatha, Kans. to Marysville. A lot of empty coal sets moving from Kansas City back to the Powder River Basin use this route.
As far as I know, Union Pacific never had any solid freight trains that were cleared for 90-mph operation. But that’s not to say that in a more civilized time when the United States Mails moved on trains nos. 7, 8, 9, and 10 - those trains didn’t exceed 90-mph when necessary to make up time. Same goes for those legendary “Silk Trains” that originated out of the Pacific Northwest.
Decades ago the Chicago-Los Angeles Super C trains operated by the Santa Fe Railway were usually powered by F45s, FP45s, and other EMD power geared at 59:18. The trains haule
The cost of speed can involve a lot more than fuel. Remember the “Bullet Train” test runs by BNSF a few years back in response to proposals by UPS. A high speed freight train to meet the UPS proposal was technically feasible but it tied up everything else on the railroad in the process. BNSF declined to bid on the UPS proposal.