Trackage Rights Operations

Since time immemorial, the New Haven Railroad and its predecessor had trackage rights over the New York Central and its predecessors between Woodlawn and Grand Central Terminal. New Haven trains entering Grand Central Terminal were odd-numbered trains since they were heading west, while New York Central trains entering Grand Central Terminal were even-numbered trains since they were heading east. In the New York Central employee's timetables New Haven trains were identified by a "Y" prefix to their train number.

Any idea why the "Y" prefix?

X and Y as in diagram,
X for north south(NYC) and Y for east west (New Haven)

Further on this: "X" trains were Harlem Trains, "Y" trains were New Haven Trains and "Z" trains were Hudson Trains.
Noel Weaver

Perhaps it's apocryphal, but wasn't it 'Y' for Yankee as in New England as in where most NHRR trains originated and terminated?