Tertius started life as a part-built Tinkerbell loco bought by a private customer with the contract awarded to Narogauge to complete the build.

When delivered to the workshops at Moors Valley in 2013, the loco was little more than a rolling chassis. Construction was done alongside another new build named Frederick.

Tertius is based on the styling of Talos but with a covered cab and rear stanchions, however this can easily be converted to run without the cab roof on a sunny summer day.

For the observant, Tertius differs from Frederick by the inclusion of buffers as well as the more usual centre coupler.

Tertius usually runs on a private railway but has visited Moors Valley on many occasions.