Transport for London (TfL) thought the Night Tube would be popular; it didn't realise quite how popular.

At a meeting of the London Assembly Transport Committee to discuss progress of the services, London Underground's managing director Mark Wild said that at the moment, passenger numbers were 50 per cent higher than forecast on the Night Tube.

The services were first launched on the Victoria and Central lines on 19 August and then joined by the Jubilee on 7 October. The Night Tube will also come to the Northern and Piccadilly lines by the end of the year.

Read more: Here's when the Night Tube starts on the Piccadilly line

Chair of the Committee Caroline Pidgeon noted Transport for London (TfL) expected the Night Tube to run at a loss for first three years of operation at a cost of £24.6m. Chief operating officer Steve Griffiths said it was possible they'll break even earlier off the back of higher passenger numbers than forecast, though it would "depend on the mix of revenue".

"But clearly it can only be more positive that we said it would pay back in three years and we're seeing greater demand than what we planned in the business case," he added.

Wild added that as the Night Tube was envisaged as a network, it would only be once the five lines were in operation that they'd be able to answer that. "But it's enormously encouraging," he said.

To date, the noise pollution concern with the Night Tube has also been lower than anticipated. Wild said: "We have only had 37 unique complaints on the Night Tube so far, having said that we still have a few hotspots." The east end of the Central line in particular, and Wild acknowledged "the close proximity to buildings on surface is a challenge".

The 37 complaints haven't been broken down yet, so some could be on behalf of a street and potentially clusters of local action groups.

Read more: Sadiq Khan has named London's first night czar

In terms of future roll-out, Wild said the reason TfL can't now expand the Night Tube straight onto the District, Hammersmith and City, Metropolitan lines was because "we're doing open heart surgery replacing the signalling systems".

The weekends are being used to install the brand new signalling system, which should be in operation from 2019. "I predict the Night Tube could be expanded to other sub-service lines after the signalling work," he added, but stressed it was important to assess how the five lines progressed to have evidence for the mayor that future roll-out could be beneficial.