uh, level 3 exhibition. Though I have to step in here.

A light load for a heavy warhorse is 300 lb. With 275 from Train alone, he + equipment will slow the horse down from the start

Once the horses strength is lowered to a mere 9, it's carrying capacity will be:
light load 90
medium load 180
max load 270

Means that after the ray of enfeeblement the horse can't move at all as long as you mount it