At the end of each verse of the song it states that the mule and boatman "know every inch of the way, from Albany (start) to Buffalo (end).

Low Bridge

I've got an old mule and her name is Sal

Fifteen miles on the Erie Canal

She's a good old worker and a good old pal

Fifteen years on the Erie Canal

We've hauled some barges in our day

Filled with lumber, coal, and hay

And every inch of the way I (we) know

From Albany to Buffalo

Chorus:

Low bridge, everybody down

Low bridge cause we're coming to a town

And you'll always know your neighbor

And you'll always know your pal

If you've ever navigated on the Erie Canal

Get up there Sal, we've passed that lock,

Fifteen miles on the Erie Canal

And we'll make Rome before six o'clock

Fifteen years on the Erie Canal

One more trip and back we'll go

Through the rain and sleet and snow

And every inch of the way I (we) know

From Albany to Buffalo.

More Info: en.wikipedia.org