I guess some of the employees showing people how to find Arlanda Express at track 10 normally staff the ticket office of Arlanda Express which is temporarly closed due to reconstruction.

An interesting thing to notice if one likes to travel to Arlanda by train from Stockholm C somewhat cheaper than by Arlanda Express. Since SL-trains are not running to/from Arlanda until sometime in October, because of the accident in May, one can travel to/from Arlanda on SJ Regional trains, i.e. not Intercity nor Fast trains ("Snabbtåg" in Swedish) and on all Mälartåg's trains with a valid SL-ticket. Meaning, if you have a valid SL card you pay only the station fee at Arlanda C of 132 kr - if you don't have a valid SL ticket you have to purchase that as well for 39 kr before boarding the train, meaning 171 kr in total. Trains with SJ Regional or Mälartåg to/from Arlanda are not as frequent as Arlanda Express but during most time of the day there are at least two per hour, sometimes more. Late evenings there are none. The journey with SJ Regional or Mälartåg between Stockholm C and Arlanda is the same as with Arlanda Express.

This information is posted on SL:s website (in Swedish only): "Biljettsamverkan med SJ Regionaltåg och Mälartåg gäller mellan Uppsala C och Stockholm C i båda riktningarna. Det betyder att SL-resenärer med giltig SL-biljett får åka med SJ Regionaltåg och Mälartåg på den sträckan. Vissa av dessa tåg stannar vid Arlanda."