There's a simple answer and a very complicated one.
The simple one is that, going north, the present light rail line leaves the old B&A/WB&A right of way at about Patapsco Ave. station.
Well, not really. Actually, it uses interurban ROW to a point just north of Waterview Ave., over a line abandoned in 1935. A bit of rather tangled history is necessary.
Up to 1921 there were two parallel rights-of-way between Linthicum and Westport, both electrified. The easternmost was originally built in the 1880s as the steam-powered Baltimore & Annapolis Short Line, most commonly known as the Annapolis Short Line. (It was this line that later became the Baltimore & Annapolis.) The ASL's line ended at Cliffords, just north of present Patapsco Ave. station, where it connected with the B&O's Curtis Bay branch. From there it operated into Camden Station over the B&O, using the B&O's South Baltimore branch to Carrolls and mainline into Camden.
In 1908 the WB&A line was completed a slight distance to the west. Going north, the WB&A line crossed through the center of present BWI Airport and began paralleling the ASL at Linthicum. (The light rail line is on the Short Line ROW at its Linthicum station while the former WB&A ROW is above its grade immediately west. The restored Linthicum station to the north at Maple Ave. is the ex-WB&A station and its west side faces the old WB&A alignment.) The two lines then ran parallel through North Linthicum and Baltimore Highlands as far as the point where the Short Line joined the B&O at Cliffords.
The WB&A then turned west, paralleling the B&O a short distance to its west/south, through English Consul (a development now chopped in two by Patapsco Ave.), turned north to underpass the B&O Curtis Bay branch (the railroad bridge is still there) and passed through the west side of Westport in a combination tunnel and cut, which underpassed Annapolis Rd. (The West[port section of the former WB&A ROW is now occupied by the B-W Parkway; the old tunnel still exists, and its southern portal is visible just north of the parkway's Annapolis Rd. exit. Watch for it when you're driving north on the parkway.) It then paralleled the B&O mainline past Bush and Bayard Sts. on a long steel trestle, which overpassed the B&O at Scott St., where the WB&A line came down onto Scott St. (The WB&A's Scott St. substation survives on the NE corner of Scott & Ostend Sts.)
Now...more complications. The Annapolis Short Line electified in 1908, at the same time the WB&A opened. When it did, the B&O was wary of a high-voltage overhead line over its tracks between the Clifford interchange and Camden Station. (Both the WB&A and Short Line then used 3300-v.a.c.) So it built a new line for the Short Line between Cliffords and its mainline at Russell St., paralleling the South Baltimore branch through Westport. This line ran immediately west/south of the Curtis Bay and South Baltimore branches, underpassing the Curtis Bay branch along the way.
Got all that?
Then came a new complication. In 1921 the WB&A absorbed the Annapolis Short Line. Afterward, Short Line trains were routed over the WB&A between Linthicum and the WB&A's new Baltimore terminal at Howard & Lombard Sts., now the site of the Holiday Inn. At the same time, much of the old Short Line track between Linthicum and Westport was abandoned, although a section was kept intact between Baltimore Highlands and the B&O Cliffords interchange to handle freight to & from the B&O. Originally, too, the "new" (B&O-built) Annapolis Short Line ROW between Cliffords and Westport also was retained for WM freight interchange, although this segment was abandoned in 1935.
The point of the windy digression is that the light rail route between Linthicum and Westport uses pieces of both the old Annapolis Short Line and WB&A rights-of-way. As I mentioned, the light rail's Linthicum station is on the old SL alignment (which later became the B&A). Going north, it continues on the ASL ROW to a point north of Maple Rd., where it moves onto the original WB&A ROW. It then continues on the WB&A ROW to through North Linthicum and Baltimore Highlands, and along this section you can see the old ASL grade immediately to the east, including its Patapsco River trestle piers. At Baltimore Highlands, the light rail line then moves back onto the ASL ROW, through the Patapsco Ave. station, and continues on the "new" ASL ROW to Westport, passing under the present CSX line using the bridge originally built in 1908 to carry the B&O over the SL. (Until the light rail was built, this bridge had remained intact, although the old SL line under it had been long abandoned.) Through this area, the abandoned WB&A alignment can be intermittently seen to the west. The WB&A's old underpass is just a short distance west of the presentlight rail underpass.
The light rail line finally leaves the old SL ROW at about Waterview Ave.
Now, aren't you sorry you asked?