Nothing regularly scheduled on the Fitchburg. As D mentions above there are regularly scheduled "extra" moves on some lines because the layovers aren't big enough for the amount of trains needed to run service, but the new Wachusett layover has an unused track so it's not necessary. These trains do have a schedule and assigned train number, but they aren't publicly listed because they aren't trains the public can ride.
I have heard rumor that PTC testing will be starting on the Fitchburg soon, so there may be an extra out running for a few months while they are testing. If they follow what they did on the Lowell Line they'll be at night and possibly on some weekends.