The cache is a lock&lock box that contains logbook. It is a BYOP so you have to bring the pencil yourself. But surely you have one, don't you?
PLEASE PUT IT BACK EXACTLY AS YOU FOUND IT AND DO NOT TRY TO HIDE IT BETTER!
As always when it comes to railway stations, be very carful and avoid getting near the tracks.
Mjölby Central Station is one of the main Railway Stations in Östergötland. You pass here on your way from Stockholm to Malmö, but also the inland railways that pass Motala pass this way. It is a big station for commutors (Östgötatrafiken) and in between Tranås, Motala and Norrköping.
Little is know about its history... and I will try to get some information and add it to this description.