The Cornwall Railway was built to the 2.14 m (7 ft 0¼ in) broad gauge. It ran from Plymouth to Falmouth. The section from Plymouth to Truro opened in 1859 when the Saltash Bridge was opened. The extension to Falmouth was completed in 1863. The original section remains open as part of the London to Penzance main line, the Truro to Falmouth line being operated as a branch. It was leased by the South Devon, Bristol & Exeter and Great Western Railways until 1889, when the Cornwall amalgamated with its three associate companies.

A realignment of the line in 1908 caused three new viaducts to be needed. One of these was at St Germans, a small village on the west bank of the river Tiddy. It had been one of the rotten boroughs, electing two members to the unreformed House of Commons until the Reform Act 1832. It was originally the seat of the Bishop of Cornwall before the see was combined with that of Crediton in 1042. Today the Bishop of Truro's deputy is known as the Bishop of St. Germans in acknowledgement of this, although he has no specific links with the village.

The stone viaduct is built over the muddy estuary of the river Nidd and its piers are 18 m (58 ft) below the water. Its maximum height is 32 m (106 ft), it has seventeen arches, and is 298 m (978 ft) long. It is a splendid sight. It is close to St Germans Station, which is a well preserved example of a Cornwall Railway station, with single storey plain buildings, rendered and with flat awnings. It is a Grade II Listed Building.

Half a mile closer to Plymouth another muddy creek is crossed by the Nottar Viaduct. Slightly smaller than its neighbour, it is nevertheless 281 m (922 ft) long and has twenty eight arches.