what options do we have for fibre link between sites?

we have 3 sites, 1 at the top of a mountain, 1 at the bottom, and one in the middle

the bottom and top site is conected via fibre optic cable that runs up the side of our mountain train track

we want to network enable the building located at the middle of the mountain by taking a feed from this existing fibre link

would we be able to add some connection/converter midway on the fibre link that would allow us to feed the network into the middle building? (there is RJ45 cabling already resident in the middle building.. used just now for telephony)

if we could just get a feed from this link somehow and get it into a switch/router that would solve our problems of getting the outdoor staff based there onto our company network

any advise please? on the cheapest and easiest way to get this done?

ps this fibre link was installed before my time within the company and thus no paperwork exists either