thegreeneworks

This weekend the M is running via the J line and making J stops in Brooklyn and Manhattan

Why does the MTA differentiate/redirect service in this way rather than running something like “special J” train with modified service or car sets? I wonder especially because the M and J already share stops along the line.

Maybe asked another way, why does a train running the service pattern of another not just go by that service?

Apologies if this is a silly wording and TIA for entertaining my curiosity.