Without being too flippant, life expired track is simply when some or all of the track system is beyond economic repair, and it is easier to replace.
There are 5 main components (top first)
Rails
Fastenings
Sleepers (bearers in point work)
Ballast
Formation including drainage
Rails can last a long time - I've pulled out rail over 100 years old. Equally, some needs renewing every few years, particularly on rails with high tonnage, high speed and high cant deficinecy (eg WCML and ECML). Rerailing is classed as a renewal once you go above a certain length (can't remember how long).
Replacing fastenings is a maintenance job. However some fastenings are more prone to failure than others (eg Pan 8) and are considered life expired, and the whole track system will be replaced at the same time.
Sleepers - if the odd one breaks that's a maintenance job. Timber sleepers, particularly softwood, will normally rot more quickly than a concrete sleeper will decay. However some types of concrete sleeper are known to decay more quickly basically due to either their reinforcement wires corroding, and/or the concrete mix being less than ideal. Again, this will happen more quickly on high tonnage / high speed routes.
Ballast loses its angularity over time, partly through small movement under the passage of trains, and partly through tamping. Tiny pieces break off, creating fines (essentially sand) which clog the ballast causing it to lose its drainage properties. Rounded ballast is also not so good at locking sleepers into place, so more minor movement occurs and then the problem escalates. There are some locations around the country where limestone ballast was used; this is less hard than granite and will become, a problem much more quickly.
Formation life depends on the geology underneath. Decent rock or sand formation is great, clay and peat / fen is not. Clay is well know for pumping up through the ballast causing a 'wet spot' which causes all sorts of geometry and track defects. Digging out a wet spot on a wet Tuesday night in February is (in my experience) the hardest job on the railway. Any replacement of ballast and formation more than an occasional wet spot is a renewal.
Putting all the above together, life expired track can be any or all of the above, but usually means rail, sleepers, ballast and fastenings together, with formation / drainage done in about 20% of cases.
However once you have to renew the ballast, it is almost always more efficient in whole life terms to renew the sleepers and rails as well, as the cost of the new materials is a relatively small proportion of the job. In fact it is usually actually cheaper in initial cost terms to replace rails and sleepers with new than save the old ones. The process of dismantling and temporarily storing the old stuff takes longer and needs more plant and manpower than it does to simply cut the old track into 20 metre panels, chuck it on a train, and get it away. If the old sleepers have any life in them they can be reused elsewhere; rail will typically be scrapped unless it is in in very good condition. (The scrap value is generally more than the cost of unloading and reloading the rail at a depot, plus doing the necessary ultrasonic checks to ensure the rail is fit for reuse).
