Their work forces are of similar size and composition. Employees perform roughly the same tasks: operating trains, punching tickets and maintaining tracks.

And yet in one area — debilitating illness and injury — the difference is so vast as to almost defy medical explanation.

One example: disabilities resulting from arthritis and rheumatism. From 2001 through 2007, Metro-North had 32 cases, compared with 753 at the L.I.R.R. In one year, Metro-North had just 2 cases. The L.I.R.R. had 118.

For certain diseases of the musculoskeletal system, like a herniated disc, Metro-North had 49 cases. The L.I.R.R. had 850.

No one at the two railroads, at the transportation authority or at the Railroad Retirement Board could explain these gaping differences, nor were they even aware of them.