@drdoombot Great answer! They actually managed to get Grand Central station platform air conditioned on the 4/5/6 lines. They are working towards getting some (not all) of the other major hot spots done. Like all things in MTA, it takes time to get these things done for whatever reasons.
Adding more trains is something they have been doing. That raises another problem in itself though. More trains are added during revenue (rush) hours to accommodate the passengers. However, it causes “train traffic”. I’m sure you’ve heard the announcements on trains. Trains running back-to-back doesn’t allow for any running room and thus, slows trains down. The lines that have the worst train traffic in the IRT subways are the local 1,6,7 lines.
You also have to take into account any emergency repairs or maintenance along the lines will slow this down further. Regular maintenance and repairs are done at night and between rush hours. Only emergency work can be done during rush hours.
The BMT lines are wayyyyyy bigger than any IRT lines and this allows them plenty of running room and also creates a longer amount of time you have to wait on the platform for the next train because of the bigger headway between trains.
Fact: During the late evenings, after express services shut down, the headway between trains in the IRT (numbered trains) are approximately 10 minutes. The head between trains in the BMT (alphabet trains) are about 20 minutes. So if you happen upon a platform late in the evening and just missed one, expect to wait 10 minutes for the next one if you are taking an IRT line or 20 minutes for the next one if you are taking a BMT line. This is the general wait times without any maintenance work going on. Work is done almost every night. The type of work being done will fluctuate these wait times.
Hope this was helpful.