Why Locomotives Can Pull So Much
What this covers
In today's video, we go over all the things that allow locomotives to pull trains many times their weight.
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Locomotives can pull enormous loads because they combine extremely low rolling friction between steel wheels and rails, high weight for traction, powerful motors, and a mechanical system (draft gears) that overcomes static friction by gradually adding weight rather than pulling the entire train at once.
- Rail-wheel contact area (~dime-sized) creates far lower friction than tire-road contact, enabling massive pulling capacity
- Draft gears allow locomotives to overcome static friction incrementally by pulling one car at a time rather than the entire train at once
- High locomotive weight (428,000 lbs) combined with powerful traction motors provides the torque needed despite low friction
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It would be near impossible for a locomotive to pull an entire train at once from a dead start because the combined static friction would be too great, but if the locomotive starts by pulling just one car at a time and gradually adds weight, then a single SD40-2 locomotive can pull miles worth of rail cars.
“it would be near impossible for any locomotive to pull an entire train at once from a dead start but if the locomotives start out by pulling just one car at a time and gradually adds on weight then all of a sudden you can have a single sd40-2 full of miles worth of rail cars”
When a train comes to a complete stop, the static friction from hundreds of wheels makes it extremely difficult for locomotive engines to get everything moving, despite the extremely low rolling friction between cars and rails.
“for simplicity's sake we're just gonna assume this train is starting out on a straight track with a zero percent grade all the rail cars that make up the train weigh exponentially more than the locomotives and because the trains at a dead stop the static friction from hundreds of wheels makes it extremely difficult for the engines to get everything moving while it's true there's extremely low friction between the cars and the rail it doesn't mean there's no friction in that small amount of friction can quickly compound”
The contact area between the top of the rail and the bottom of train wheels is only about the size of a dime, creating a very low friction environment that allows locomotives to pull massive loads.
“the contact area between the top of the rail and the bottom of the wheels is only about the size of a dime”
On exponentially long trains, the engine might be moving at a couple miles per hour before the end of the train even starts to move due to the slack created by draft gears gradually adding weight one car at a time.
“this is what rail fans and railroaders call Slack on exponentially long trains the engine might even be moving at a couple miles per hour before the end of the train even starts to get going”
A truck trying to pull a million pounds of semi-trailers would fail because the static friction between the trailer tires and the road would be too much to overcome due to excessive contact area, whereas an electric F-150 successfully pulled a million pounds of railroad auto racks because rail cars have minimal friction.
“if this truck were to try and pull a million pounds worth of semi-trailers it would never work the static friction between the trailer's tires and the road would be too much to overcome because there's too much contact area”
Draft gears allow locomotives to overcome static friction one car at a time, which is why a single locomotive can pull thousands of tons of rail cars as if it were nothing.
“since draft gears allow locomotives to gradually add weight one car at a time this also means that they're only overcoming the static friction of one car at a time given locomotives the ability to pull thousands of tons like it's nothing”
The ability of locomotives to pull enormous loads is a combination of: overcoming static friction by adding weight gradually, taking advantage of extremely low rolling friction, and using exceptionally powerful traction motors.
“so there you have it it's all a combination of overcoming static friction taking advantage of extremely low rolling friction gradually adding weight and exceptionally powerful traction Motors that give locomotives the ability to pull things exponentially heavier than they are”
Draft gears are located behind each coupler on a train and allow weight to be added gradually by letting the coupler slowly move in and out of a large sleeve, enabling locomotives to overcome static friction one car at a time.
“all this thanks to the draft gear located behind each and every coupler on the train draft gears allow weight to be added gradually by letting the coupler slowly move in and out of a big sleeve”
A typical SD78s locomotive weighs 428,000 pounds with 12 points of contact with the rails, meaning each wheel's contact area bears approximately 35,500 pounds of weight.
“let's look at a typical sd78s which weighs 428 000 pounds with 12 points of contact with the rails this means that each of the wheels contact areas have 35 and a half thousand pounds crushing down on an area similar to the size of the dime”
Locomotives are equipped with some of the most powerful traction motors on the market, which provide immense torque necessary to overcome friction and pull trains.
“locomotives are also equipped with some of the most powerful traction Motors on the market which provides immense torque”
Static friction is the force that opposes the potential momentum of a stationary object, and any stationary object will remain in place until the static friction is overcome.
“static friction is the force that opposes the potential momentum of a stationary object in any stationary object will remain in place until the static friction is overcome”
A person can move a single rail car or locomotive nearly unaided due to the extremely low friction between rail equipment and rails, though this is dangerous and should not be attempted.
“it's possible to move a single rail car or locomotive nearly unaided with that being said don't know one of y'all go around trying to do this it's far too easy to get hurt or end up with a runaway car”
Draft gears can be audibly heard in action when trains are accelerating and decelerating, as the coupler slowly moves in and out of the sleeve.
“you can even hear draft gears in action when trains are accelerating and decelerating”