Anybody within reach of the emergency cord could do it.
There would be a whoosh of air on the car where the brake was activated, and a bang on every car as the triple valve went into emergency.
Depending on conditions, decelleration rates and efects could vary. On slippery rails the wheels could lock and the train could slide at some speed for some distance, possibly emitting sparks frpm the wheels. On good rail the decelleration would be faster, but the brake shoes might get hot enough for an odor to be detected. In most cases, the train might take over a mile to stop, so the rate would not throw people from their seats. Standees may get jostled a bit, especially on the final stop. Nothing like the effect of a collision though.
Gerry. STM/BSRA
The next stop is Washington. Change for Forest Hills Trains on the Winter St. Platform, and Everett Trains on the Summer St. Platform. This is an Ashmont train, change for Braintree at Columbia.