I used this line recently. There is a bus rail replacement currently between Creueta and Benidorm. The line is pretty scenic once you change onto the diesel tram train to Denia. Has there ever been trough running from Alicante to Benidorm?
A new tunnel was built to avoid the congested single line alignment of the original route at this point around 2018. The line to Porta del Mar closed in 2013 (having had a very poor service after the city-centre extension opened) but has now reopened with a decent timetable.If it was a single track joining you on the right, travelling towards the tunnel, that is the original alignment from La Marina station, now used by route 5 back to Porta del Mar by the main pier. See
maybe out of use at present?