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Isn't there some static limit to how long a train can be? On a 1% grade with a 15000-ton train, the first knuckle is loaded with 150 tons statically, which sounds like a lot. It seems like you wouldn't be able to practically enjoy the power of 9 locomotives because you wouldn't be able to pass that much force through the knuckle.

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It's not the pull that leads to problems. It's the braking. The engines in front might have come to a full stop before the last cars a mile or more back have even gotten the message that they need to decelerate. Do that on a curve going down a hill and the cars in motion just keep going, usually into a river or some buildings beside the tracks. The longer the train, the more the whip cracks.

trains have air brakes connecting all the cars... theres no delay.

They put engines in the middle and at the end to avoid running out of knuckle capacity. It works most of the time, except all those engines pushing and pulling tends to increase the chance of derailment.

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