Elgin Street North briefly transforms from a road to a pedestrian tunnel and back to a road again, which is unfortunate if you're driving a car. Even more curious is every map I own & even my GPS suggest you can drive through it! Well, it might be a tight squeeze if you tried. During the summer of 2009, area residents and the City of Cambridge spruced it up to be more pedestrian welcoming.
Please note this pedestrian tunnel runs underneath live railway tracks. FOR YOUR OWN SAFETY, OBEY THE WARNING SIGNS AND STAY BEHIND THE FENCES AWAY FROM THE TRACKS.
The cache is not located at the posted coordinates! To find the cache, some close inspection of the inside of the pedestrian tunnel will be required. Some of its construction characteristics will need to be considered with a little bit of math to calculate the cache's true coordinates. The tunnel is constructed from corrogated steel sheets formed into a giant pipe which is lag bolted into concrete.
To find the north coordinate offsets, you will need to count the number of corrogated "ribs" that circle around you in the tunnel. Count the number of fully complete ribs and call this A. Count the number of incomplete ribs and call this B. Tip: imagine the incomplete rib going around you and count that as one. Next, solve this equation (if the brackets scare you, just solve it reading left to right 
):
C = ( ( ( (A - 1) / 2) - B) x 4) + B. The north coordinate can be calculated by subtracting the value of C / 1000 from the posted north coordinates.
To find the west coordinate offsets, you will need to look at the lag bolts in the tunnel. Count the number of nuts and call this D. Where the tunnel sections are bolted together, count the number of bolts in this (single) complete rib and call this E. Tip: there are only a few places in the tunnel where sections are joined, and lots of bolts are used. Next, solve this equation:
F = (C + D) / E. The west coordinate can be calculated by subtracting the value of F / 1000 to the posted west coordinates.
*** The math should be easy. If it's not, check your counting.
30-Apr-2018: Final had to be relocated due to deforestation of area: please subtract 0.018 from northing and 0.006 from westing. The geochecker still reflects the original puzzle, and doesn't include this correction.
For those arriving by car, there is plenty of curbside parking near the posted coordinates on adjacent suburban streets or alternatively in Soper Park. While you're in the area, be sure to enjoy the view of the stream and the old railroad archway northeast of the pedestrian tunnel.
You can check your answers for this puzzle on GeoChecker.com.