January 28th 1855 saw the official opening of the Panama Railroad, which linked the Pacific and Atlantic Oceans.
This may not be as impressive as either cutting the Panama Canal was, or building railways across the continental United States, but it still was a significant feat, one which saw the death of between 5 and 10 thousand people. No official record was kept.
The railway was first surveyed in 1849 and construction began in 1850. The initial estimate for the cost was US$1 million. The actual cost was $US8 million - a little bit of an overrun - however, by the time the line was officially completed and opened, trains had already been running over the completed stretch and had managed to recoup a third of that cost.
Image: By Fessendon Nott Otis, Colored lithograph by Charles Parsons [Public domain], via Wikimedia Commons (commons.wikimedia.org/wiki/File:CulebraSummit.jpg)