The Worcester Main line has some pretty steep grades. A few weeks ago, a P&W coal train with about 80 cars could not even get started at New Bond st with 5 P&W GE's. The crew had to pick up two more engines in Ayer and come back.
The main reason you see so many engines despite the slow speeds is many of the engines are not performing properly. Many have ineffective wheel slip systems, loading issues, transition issues, etc. Properly maintained, modern units would make a big difference in the number of units you see.