This official RDG document indicates there is one but there is no human presence at all at Acton Main Line. The document is dated 2015 but there was no ticket office even then.
https://www.raildeliverygroup.com/files/Publications/services/rsp/TSA_schedule_17_July_2015.pdf
https://www.raildeliverygroup.com/files/Publications/services/rsp/TSA_schedule_17_July_2015.pdf
