These are tools, which mainly consist of a steel hollow tube with a steel rod inserted into it, which can allow the tube to roll in position. Alternatively, steel rollers mounted on a holding arrangement may also be available in the open market. These can be procured from open market / locally manufactured.
These, when placed beneath a rail, free of any fittings, can enable free expansion / contraction of rail above it.
They are used on every 15th sleeper on rail seat under foot of rail for easy longitudinal movement of rail. If the track is curved,the side support rollers are also used adjacent to the under-rollers on the outside of the curve, to maintain control of the rail laterally.