Ironically their timing is just a little off from the true completion date of the transcontinental railroad. The section from Sacramento to the Bay Area was completed on September 6, 1869. There has been some question as to where the final spike was driven but I believe it was at the San Joaquin River bridge just east of Tracy. The Niles Canyon Railway operates on the west end of that connection. Unfortunately from the NCRY end of track to the San Joaquin County border the track was torn up. But if they really wanted to reenact the final completion, the spot by Tracy is still on active track.

A note on the final spike location: I have heard it proposed that it happened in the following places:

By Hayward - Unlikely since the line from San Jose to Oakland through Niles was already in service.

In Niles Canyon - This is driven by the fact that the original Western Pacific Railroad built the line to roughly milepost 33 in the canyon before running out of funds. The CP bought the WP (original one, not the one we know and love today) and completed the proposed line over Altamont Pass.

At the San Joaquin River bridge - the most likely place for the final spike. Building the bridge would have been one of the most time consuming parts of the construction process. It is most likely that the tracks were extended south from Sacramento to Stockton - a quick way of grabbing valley farming business, then down to the crossing. From the west the line was already graded most of the way through Niles Canyon when the CP bought the WP so building east through the Livermore Valley would have been relatively easy and another revenue generator. Altamont Pass would have been a little challenge but they were able to build the line using cuts and fills with almost no bridges. So it would have been pretty straightforward construction.