Traffic on the canal increased rapidly, and the summit level was lowered in the 1790s, cutting out three of the locks at each side.

At the same time, the remaining three Smethwick locks were duplicated, to speed the passage of boats.

[1][2] As trade continued to increase, the Smethwick summit caused problems, both of water supply and of congestion, and so the company commissioned Thomas Telford to construct a new main line.

This would follow more recent practice, using cuttings and embankments to follow a much straighter path, and the resultant main line, now called the new main line, shortened James Brindley's winding contour canal by around 7.5 miles (12.1 km) to 15 miles (24 km).

The deck is of cast-iron plates with raised ribs 2.5 inches (6.4 cm) high cast on their upper surfaces to help retain the earth filling which forms the footway.

The bridge abutments and wing walls are of brick with stone facings at the corners.