How are tank cars unloaded? - Trains Magazine

GATX, the tank car people, have a system called "TankTrain."  It consists of a series of individual tank cars that are semi-permanently coupled together.  At the top of each tank car end, a product flow hose connects one tank car to its adjacent one or ones.  The idea here is that a customer needs to attach only one hose to the TankTrain in order to add or drain product to/from all the cars.  The system is designed for liquid shipments.

Adding product seems like a no-brainer.  Start pumping the liquid into the first car and when that one fills the excess product will spill over into the second car and eventually to the third, fourth, fifth, and however many cars constitute a TankTrain set.

To get product out, though, it would seem to require a compressed gas that would have to be injected into the one end of the TankTrain set while the opposite end drains the entire set of cars.  Injecting that gas under pressure strikes me as very energy intensive and could therefore negate the convenience of having only one hose connection for unloading.

Also there's the issue of minimizing product spills during derailments.  Unless the TankTrain is hauling fresh water, I should think it would require some kind of shutoff valve attached to each end of the product flow hoses.

I've closely observed a lot of freight trains in recent decades, but I can't recall seeing any more than a very small number of TankTrain cars.  Does anyone know if they're being used anymore?

For further information Google "TankTrain".