If you want the short answer - in the western part of the US, about 100-150 cars. Ok, we're finished.
But if you want to know the reasons why, read on!
A long train needs both pulling and pushing power (I'll explain why later.) Locomotives supply that power.
Front locomotive horsepower + helper locomotive horsepower = total horsepower
Total weight / total horsepower = tons/hp
A rule of thumb is to have no less than .6-.8 hp/ton
For example, can we handle 100 cars with 4 locomotives? Let's assume:
- Cars weigh 90 tons each x 100 cars = 9,000 tons
- Locomotives weigh 150 tons each x 4 = 600 tons
- Locomotive 5000 hp each x 4 = 20,000 hp
20,000 hp ÷ 9600 tons = 2.1 hp/ton So yes, we have enough power for this string of cars.
8 locomotives pulling 120 freight cars
On a train the brakes set when pressure in the air brake system drops. That means if you have air leaks or a car breaks free, the cars will stop.
When a long bunch of cars ( 1.5 miles or 9000 feet ) are hooked up, enough air escapes from the connections that it's impossible to move them reliably.
This is a French term that wine enthusiasts use to describe the complex interactions of location (soil, sun time, water, etc.) on a wine.
I'm using it here because a combination of the same sorts of things affect a train. Hills, valleys, curves, wind, weather and track all influence the total length of a train.
In addition, the train usually shouldn't be longer than the siding that allow it to pull off so other trains can pass.
Question 2 Why does a train need power in both front and back?
Because train engineers have learned (the hard way) if you put all the power in front and try to pull a long train around a curve, you can pull it straight across the gap and into a valley or lake!