Its four now, between wood and concrete. To be fair, I wouldn't think there is any more risk from heat than what there is from a few voiding sleepers.
As long as its not voiding and no slurried beds, the risk would be no more than any where else.
(Network Rail maintenance team leader of 19 years).
Anyway, I hate the ******* things! Once the bands are broken, the packs fall all over the shop, they wont stack and wont stay stacked if even you get that far.
Forget about transporting them else where once they have been machine handled once! They are also a pain in the arse to maintain, only packable by a tamper! A joy to the bean counters but a nitemare to every one else. The skurge of the railway IMO.