The Montgomery County Department of Transportation (MCDOT) will begin construction on the first section of the Met Branch Trail (MBT) in downtown Silver Spring this year.
From the video WABA links to, it appears that Phase I is the trail from Montgomery College to King Street along Fenton and Phase II is from there to Selma along King and the railroad tracks. Those sections will be 12 feet wide with 2 feet of shy space on either side (12+2+2), except where CSX retaining walls limit it to a narrower width.
The trail on the bridge over Georgia Avenue is currently planned to be 8+2+2, but it could be widened to 12+2+2 for an additional ~$650,000. The section near the old train station, where a detour is being designed and negotiated with MPI, will likely be 10+2+2 so there was a suggestion that the bridge be designed to this width as well, at a lower cost. MPI wants to see a narrower bridge, though the reason for this wasn't stated.