Before this thread gets sidetracked into a discussion about how or if Metro-North should rebuild the Piermont Branch for suburban train service, I contacted the member who posted the photo of the E unit leading the train into Suffern.
He doesn't remember the approximate date or time but he does think it was a Saturday. My April 1966 timetable is home (I'm at my girlfriend's) but later in the week I'll look to see what the schedules were on Saturday.
Maybe we can figure this out yet.
Btw, I looked at a document relating to an accident on the Erie, hopeful it might have a diagram that would answer the question that began this thread -How did EL get terminating westbound trains into the yard at Suffern? It didn't have a diagram but I do know some people who worked there during the 1960s. I'll ask around.