Allow me to explain.
I drive over Bozeman Pass pretty often and a lot of the time I see freight trains going up or down it, in one direction or the other. The grade is steep enough that they are crawling when going up…I could probably outrun or even outwalk them.
Most of these trains are very long, at least compared to the lengths of other man-made things that intentionally move, about a mile long and often more.
So my question: when traversing a pass, there will be a big chunk of time when the back of the train is struggling to make it up while the front of a train is trying like hell to not cannonball down. And of course the difference will be dynamic, as more train gets over the pass. Is this something crews have to manage, and if so, how?
Like, is there a formula or something or is it more of an art?
One notable solution is the Tehachapi Loop.
Yes they actively manage the changing uphill & downhill portions of the train. There are rules of thumb but it’s a lot of art and experience too.
Certainly not at the same time. If the couplings are strong enough for an engine at the front to pull up the rear of the train before it crests, then they’re strong enough to balance the load you describe.
Anticipating the load that comes on and off due to grades and acceleration affecting the slack action between the cars is a big part of driving a train. I’m was once told that applying too much power in the wrong circumstances will snap a coupling as if it was a rubber band.
The trains I see here going over the mountains have engines on both ends and sometime multiple engines on each end.
It’s important to remember that railroads in the U.S. simply don’t have steep grades compared to what roads might have. The Bozeman Pass railroad mentioned above has a grade of less than 2%. [a 2% grade means it rises 2 feet vertically over 100 feet of length.]
I’ve never seen a train with engines at both ends, but I guess they could be that way. More commonly around here (Alberta), trains heading westbound over the Rockies will have a couple of locomotives in the middle of the train. Either way, the additional locomotive power helps pull the last half of the train up, while the lead locomotives and the first half of the trail are going down.
As for grades, I believe (but I am not sure) that 2% is the maximum grade allowed on North American railroads. Nowadays, anyway. The Canadian Pacific route through Kicking Horse Pass did have a 4.5% grade when it was first surveyed and built in 1885, but it was not ideal: it actually had runout lanes manned by real people whose job it was to throw the switch if a train appeared to be out of control on the downgrade. That problem was alleviated by the building of the Spiral Tunnels:
The old 4.5% roadbed became a road (Highway 1), and I’ve driven it uphill and downhill. It is not bad in a car, but I’m sure that it was terrifying in a train.
Be that as it may, it’s plenty steep enough to slow a train to a crawl and require helper engines—the most I’ve ever seen was five—hooked up in back to push it over. As I understand it, trains are significantly more affected by grade than are road vehicles. Something to do with torque, I think.
With the exception of short consists hauling two or three 737 fuselages from Wichita to Seattle, every single train through here has engines at both ends. The most I’ve seen was three in front, five in back. Once in awhile I see an extra long train with three engines in front, two in the middle, and one or two in back.
Trains are heavy. Power is force times speed. Here we mean speed uphill. So multiply the forward speed by the grade times the mass of the entire train times 9.8.
The rule of thumb for most trains was one horsepower per ton. (Imperial) So maybe 0.7 kW per metric ton. Which is not a lot of power.
Try driving your three ton pickup around with a three horsepower engine.
But there’s pushing from the back happening at the same time as there’s pulling from the front. On a plain, no problem. But when the engines at front and the engines at rear have what appear to be contradictory aims (coast/slow down vs. push harder), it seems like something in need of resolution. I’m asking if that’s so, and if it is so,how is it done. LSL says there are rules of thumb and it’s something of an art gained by experience. I’m wondering if there’s more to say about it.
It’s not that they are more affected by grade. It is something to do with torque I suppose but it’s nothing mysterious - quite simply trains have very little of it, relatively speaking.
Trains are efficient because steel wheels on steel track have extremely low rolling resistance. This means the power you require to keep them rolling on the flat is remarkably low. So you can have a relatively weak engine that only has enough torque to accelerate the train slowly, and then the minimal power to keep it up to speed - which is great until you come to a grade at which point the lack of torque is highly limiting.
This is one important aspect. Trains have very low rolling resistance compared to rubber tires.
Also, if you want to do an energy requirement calculation, at a very rudimentary level consider that essentially, ignoring rolling resistance and similar issues, you are raising the weight of the train by X feet every minute, adding to its potential energy.
Things get very challenging when the train is long enough to be going up and down multiple grades at the same time. Managing the runout and compression at several locations along the train simultaneously is very tricky.
Really? It seems pretty common here in Texas. Out West, I’ve even seen a few coal trains in Wyoming (or was it far western Nebraska?) with engines in the front, rear, and even one in the middle of the train.
If I had to guess, the placement of the engines is the secret sauce for managing trains in rugged terrain. You can push/pull from one end, and less so/more-so with the other, or vice-versa for example.
In a couple of weeks there’s a good chance I’ll end up talking to my wife’s cousin’s husband who’s a locomotive engineer of several decades experience. I’ll try to remember to pose this question to him.
Modern railroad locomotives are networked together. So a train of e.g. 200 cars with engines interspersed can be managed as e.g. 4 independent trains of 50 cars each with a loco at the front of each. Who are each following each other very, very closely. That greatly reduces the difficulty of controlling the whole.
As said upthread, trains are extremely underpowered compared to cars or airplanes. Or even long-haul trucks. Because it’s most efficient to have a small powerplant running hard than a big powerplant running at a fraction of its full output. As long as the railroad companies are OK w the pace their trains get from A to B they’re happy to pocket the fuel savings.
A consequence is they really suffer on uphills. The nature of diesel electric power is the powerplant can be at full throttle while the electric motors are pulling for all they’re worth with all that electricity, and meanwhile the train is only able to maintain 5-10mph versus the huge weight being drug up that grade.
On the downhill the diesel powerplant is idled and the electric motors are switched to generator mode (“regenerative braking”) with the produced electricity being pissed away in giant electric heater banks with giant cooling fans that occupy most of the rear half of a modern locomotive. And before you ask, yes there are moves afoot to develop locomotives with mongo batteries to capture that electricity instead and reuse it later on the flats or next uphill.
The friction wheel brakes on the loco(s) only are the next line of defense in controlling downhill speed. Obviously those have a finite heat capacity and once they’re saturated with heat, they’ll quit working just like brakes on road vehicles do. That way lies the proverbial runaway train. Oops.
The wheel brakes all along the train are next, but are subject to a lot of vagaries & inconsistencies in how and when and how hard they apply on any given car in the string. Using much force on those brakes can part the train when one group of cars’ brakes are strong and another group’s brakes are weak.
Here’s a decent article on a severe accident in 1989 caused by failure to control train speed on a major downhill: San Bernardino train disaster - Wikipedia San Bernardino train disaster - Wikipedia. This is the NTSB accident report (200 page pdf).
The interested reader can learn a LOT about railroad operations on grades from reading either or both of those.
As an aside, I really enjoyed the PC train game Railway Empire. It has you build a railroad corporation across the US and other regions, with historically accurate locomotives. to haul passengers and cargo between routes. Navigating severe grades, tunnels, bridges, etc. is part of the fun.
It’s not really meant to be an engineering simulation, but it gives you a fun bird’s-eye approximation of some of the challenges. It was a hell of a time trying to create a route through or around the Sierra Nevadas, for example.
Have any railroad companies ever attempted to have separate “pullers”/pushers at steep grades, like extra high-power locomotives who just live at mountain passes to aid trains through them? Kinda like tugboats, but for trains?