I was discussing the electrification of Tractor Trailers as an academic exercise with one of my friends the other day and we came up with a question that neither of us had a good answer for. For an ICE truck, when going down hills everyone knows that semi's use engine breaking as to not overheat their brakes. Now in a series hybrid or full EV configuration using engine breaking is impossible but regen could be an acceptable alternative. But as most Tesla owners know, there are certain situations where regen is disabled or reduced (full battery, cold battery...). Now trains to my understanding use a large resistor at the top of the locomotive to dissipate regen energy as there is no battery to store that energy into. This seams like a doable solution but maybe not ideal in a semi configuration. One solution that I thought of (dont know if its possible) is to use the EV motor in "reverse" to slow the tractor trailer while going down the mountain. Can an electric motor be used like that?