Which London Underground line has the longest continuous section of above-ground route?

The Metropolitan line has the longest continuous section of above-ground route on the London Underground.

Much of the Metropolitan line runs through outer north-west London and beyond the urban core, where the railway travels in open cuttings or on the surface. Its character reflects the line’s origin as the Metropolitan Railway, which expanded from central London into the countryside and helped stimulate suburban development.

The line extends from central London toward Amersham, Chesham, Uxbridge, and Watford. It is therefore associated with longer-distance services and larger sub-surface trains than the deep-level Tube lines.

The District line is also heavily surface-running, so it is an easy alternative answer. The distinction here concerns the longest continuous above-ground section, not the total route length or number of surface stations.