The air conditioners are, to a good approximation, just pumping energy out of the train cars that would be released anyway (i.e. the 100 Watts per passenger discussed below), plus whatever power they require themselves to run. Thus it seems very unlikely to me that train car air conditioning is a substantial contribution.

One could argue that perhaps the passengers' metabolisms are increased slightly to make up for the extra heat loss due to a cooler environment. But this has got to be totally negligible with respect to the energy dumped by a stopping train.