The December incident was when Gamble slipped and fell on the ice on the Executive Mansion steps, reinjuring his already mangled arm. The initial accident with the train occurred at the Gray's Ferry Bridge (aka Newkirk Viaduct) over the Schuylkill River immediately west of Philadelphia on the evening of August 5, 1863.
After the second accident an infection set in, which has also been attributed as being the cause of death. It appears pneumonia also set in, resulting in primary and secondary causes of death of Gamble. Strangely so little was reported on this incident that he was being mocked by his unknowing political foes as being afraid to face the legislature while he was essentially laying on his death bed.