Complex sentence: It was not deepened until 1847 (work designed by Telford's close associate, James Walker) by which time most ships were too large, and Inverness was soon connected to the Lowlands by railway.
Split into simpler sentences: It was not deepened until 1847 (work designed by Telford's associate, James Walker) by which time most ships were too large. Also, Inverness was soon connected to the Lowlands by railway.