Most subways have air conditioners on the trains which expel heat into the tunnels, thus making them hot. Before air conditioning was introduced to the Boston s

Most subways have air conditioners on the trains which expel heat into the tunnels, thus making them hot. Before air conditioning was introduced to the Boston subway system, the cars had little vents that pulled cool air into the trains when the cars were underground, but with the advent of air conditioning, the trains stay cold at the expense of the tunnels and stations.