The service on the new lines north to Harrow started on 15th June 1912.
Were these platforms used as a terminus before then and if so when did that start?
According to Joe Brown’s ‘London Railway Atlas’ (I have the first edition), the new lines opened north of Willesden Junction in 1912, along with most stations. South of Willesden Junction, the new lines opened in 1915, the new formation ending at Queen’s Park where it merged with the previous formation to Euston. Many connections are shown in the atlas; there is no evidence of any connection that would allow ‘new lines’ trains to access the older lines at Willesden to continue to Euston. Whilst the information is by necessity summarized, it appears sensible to infer from it that Willesden Junction station was used as a terminus for ‘new lines’ services from the north for around three years.