BLS Midway is at MP 0.48 which is the property line between RGVRRM and the Town of Rush Park (NYMT). The shelter building is on the NYMT side of the property line. The NYMT "Midway" trolley boarding platform is north of the building on the opposite side of the tracks. The marker on your map looks to be too far south since the sharp curve to the north, "Scanlons Curve", is MP 0.56. Midway is the south end of NYMT electrification. The shelter building is not used by NYMT.
EDIT - I looked at your map again with it zoomed out to see the whole line and I see your mark is actually on Scanlons Curve which is about 400' N, not S, of Midway. MP 0.0 is the S end of track at RT 251 so you can measure from there to be accurate. I originally mistook the curve you had marked as the first curve N of Switch 6.