Here is the rest of the quote in a letter by Van Gogh:
So it doesn't seem impossible to me that cholera, gravel, pleurisy & cancer are the means of celestial locomotion, just as steam-boats, omnibuses and railways are the terrestrial means. To die quietly of old age would be to go there on foot.
Basically, Van Gogh is saying that it's very possible that disease states are fast ways to reach death akin to riding on a locomotive, bus, or steam-boat to a final destination. But, dying of old age is getting there slowly as if walking there on foot.