LONDON—As the lunchtime crowds bustled by Liverpool Street Station on Wednesday, Jay Carver peered into a dirt pit where a skull and the bones of several unknown people lay visible.

The remains are the latest of some 400 skeletons he and his team have uncovered at this site—and he expects to find thousands more. The bones are part of a mass burial ground dating to around the 1600s that is being unearthed in the building of a new rail route through the city's center, said Mr. Carver, lead archaeologist on the project.

...