Rich, the current fleet is heavier than what they replaced. The cars, both MU and diesel average a 50,000lb increase over the previous generation of cars. The locomotives are approximately 30,000lb heavier than previous diesels.
What compounds the braking problem with the newer cars is literally the braking ability of the equipment. Since the LIRR wants the cars to reduce speed at a faster rate (3.7fps) than previous generation equipment (1.6fps), there is a higher braking rate design which in turn causes the wheels to lock up and the trains to slide. Due to the amount of trains and the close blocking necessary to run their service and still have an MAS of 80mph, they had to have a braking design that met the requirements they needed in order to accomplish what they wanted.
The Rail Road would be irresponsible if they didn't slow the trains down when there are serious adhesion problems due to leaves, snow and rain.