Got multiple emails from people saying “Hey, have you seen this yet?” I knew this has been going on but no, I hadn’t seen these particular videos yet. The videos, taken by cjd1701, show rail grinding being done on Interstate. A specialized machine called a rail grinder goes over sections of the alignment after revenue service hours and evens out irregularities in the rails caused by normal wear and tear. For MAX passengers, this translates to a smoother ride.
Grinder by day (picture from BikePortland‘s Twitter)
Videos taken by cjd1701.
Camelopardalis
MAX FAQs 
Woo hoo rail Zamboni!
Does a machine like that actually *grind* the rails as well, resulting in a loss of material over time, or does it just re-align them?
Loss of material over time. If you remember the rounded heads on the rails at Sunset when the line first opened, you’ll notice that the rails are now flat and almost flush with the concrete at the emergency crossing on the east end of the platform.