I've spent some time on this very problem looking over bike maps and I've concluded that there is really not any completely safe route between the Hyattsville/Mount Rainier/College Park area and the Takoma Park/Silver Spring area
probably the safest route is the Northwest Branch to the Sligo Trail and even that isn't so great because of some horrible at-grade crossings ... like the one at East-West Highway
... I don't know what the hell the city planners were smoking when they decided to have an MUP cross the busiest road in Prince Georges County at-grade, with only a stupid crosswalk and a "watch for bikes/peds" sign that no one pays attention to, but it must have been pretty good