The rails would be moved to a safe position before the line is handed back. However rails weigh upwards of 56kg/m, they're not going to suddenly jump around. The bigger danger of rails not being collected is to lineside staff with rails stacked in the cess.
Rails left in the four foot do however creep along with the vibration of trains. If they encounter signalling equipment such as cables or TPWS "loops", they can damage these.
buffer stops and fencing were also made from old rails.
When the GWR converted from broad gauge to standard, some of the redundant rails were re-used as posts for "No Trespassing" signs.
The father of a school friend did some metallurgical study on these rail samples back in the 1970s. One of the samples now serves as a paperweight at home.
There is still many fence posts and posts for signs (some disused) across Western made from old broad gauge rail. And some made from old bull head rail.
Also in places, old rail has been used to help hold concrete panels/boards to hold the ballast formation or banks in place.
Back to the OP question.
There are many reasons, many of which have already been mentioned:
- Brand new rail laid out for future use, either for a planned job or for emergency use.
- Brand new rail laid out for a planned job that got cancelled.
- Old rail from a planned job that has not been recovered yet, but is planned to be collected in the future.
- Old rail that was supposed to have been collected, but something happened that prevented it.
- Old rail that has been "forgotten" about.
Rail replacement is not all done the same way. What is in the job specification and hence the contract if the work is being contracted out varies. With some jobs, it is part of the specification that the old materials are removed from site.
However, if difficulties are encountered, yes, collection and removal of the old materials is often not completed. Then the company has no more time to carry out this work...
When Network Rail DU maintenance or CAPEX/Works Delivery are doing the work, they typically do remove the old rails. As the machine that brought the new rail can take back the old rail.
The overall responsibility for dealing with old materials comes down to each Network Rail Delivery Unit (DU). At one time there was a central services section (based at Eastleigh) that ran a train that collected old rail. But this was shut down due to problems.
For rail that is not CWR/LWR, typically a DU will use a 360 RRV (a Road-Rail Vehicle 360-degree excavator fitted with special attachments to pick up rail and fitted with rail wheels) and an attached trailer to take the rail to a pre-arranged collection point. It will then be loaded onto road transport. But due to budget limitations, this work is happening less often.
About the weight. For short lengths, it is possible for a team of people to lift rail. But only if there are enough people and they have the correct tools. I've seen this taking place. They transport the rail either on a P.Way hand trolley or using wheels that are attached to the rail that they are moving.