Someone did bring up this question at the meeting although no one came up with anything definitive about the structure, which I had thought was a bridge.
But I do think you nailed it here. The 1921 Bromley map below shows the train yards and you can see just above the tracks is a section of road labeled “ROAD.” You can see this same road going up the hill in the original photo to the left of the structure. This road is essentially the same path that exists today from Waldo Ave to W. 240th Street (alongside Manhattan College’s new student center).
The NYPL has a great photo of the train yard in 1926 before Gaelic Park was constructed in the field to the south:
Great detective work and thanks for sharing.