The other shoe is about to drop. First was last year's announcement that the Baltimore & Ohio Railroad, part of the Chessie System, planned to seek permission to abandon its Georgetown branch. Now the B&O says it will file the formal application a week from Thursday.

But for all practical purposes, abandonment of the 11-mile line, which was completed in the mid-1890s, already has taken place. The last train ran last June between Georgetown Junction in the Rosemary Hills section west of Silver Spring and the end of track under the Whitehurst Freeway near 30th and K streets NW.

Service was halted, according to Lloyd Lewis, a spokesman at Chessie's Cleveland headquarters, because of structural problems with a bridge in Montgomery County. But the final blow came in November when waters from the upstream Potomac River flood undermined about 75 feet of roadbed near Fletcher's Boathouse. From Arizona Avenue into Georgetown, the line parallels the Chesapeake & Ohio Canal, a national park reservation.

Toward the end of train operations, the only customer was the General Services Administration power plant, which now gets its coal by truck from Northeast Washington.

The B&0 said its application will be filed with the Interstate Commerce Commission. The procedure would permit another railroad to take over and operate the line, but the prospects of that are, conservatively, nil.

The railroad has put plans for developing the right of way on hold and has hired an appraiser, Lewis said, in hope of reaching an agreement on a land swap with the National Park Service.

Several readers over time have suggested that the line, which curves through Bethesda and Westmoreland Hills west of Washington, might be converted into an electric light-rail transit line.

It's a tantalizing thought, although it would face formidable problems: Overhead trolley wires and fast-moving trains might not be compatible with the sylvan C&O Canal, but, for that matter, the freight trains' diesels (or the steam locomotives before that) weren't, either. Too bad the old Cabin John trolley bed wasn't preserved; it could have hooked in to that. Fix That Sign

Ernest L. Schwab of Potomac complains that an electric sign at the D.C. end of Chain Bridge has been out of operation for months, and it is causing traffic havoc as drivers try to sort out their cars for a turn onto the bridge or a direct run toward Maryland. The sign above the roadway says:

USE --


Electrified, it should read either "USE 1 LANE" or "USE 2 LANES," but its message is mute.