Overnight Green Line Maintenence?

Around 2am on Friday night/Saturday AM I saw a large work crew by the Coolidge Corner station. Sparks were flying everywhere!

Is this rail grinding? What is that? What was this? Why? Is it dangerous? People were standing nearby without any real protection.

What sorts of work is done overnight?

Sounds like the rail grinder. Going to be a new fact of life for the Green Line and its neighborhood.

From what I know, the bigger problem is the noise it generates, but the main danger is sparks lighting dry brush on fire. These grinder have either onboard sprayers to douse small fires, or would be followed by support vehicles to do so.

You're most likely right. I think there was something going on with the track rehabilitation on the Green Line, and it was track grinding. This is to hopefully improve the speeds and even allow more flexible Type 8 operation on that area.