The main three things are: 1) very complicated and drawn out design/approval processes with many expensive consultants involved. on network rail you would not

The main three things are: 1) very complicated and drawn out design/approval processes with many expensive consultants involved. on network rail you would not believe the amount of studies they have to do, including testing the light levels everywhere in a carpark to make sure there are no dark spots, amongst a billion other small things. there has been endless regulation forced on the railways/tfl, which on one hand is good for safety but on the other has meant even a tiny project needs a trillion risk assessments done, which typically get outsourced to specialist consultants, who have a massive incentive to find problems so they can retest. 2) lack of competition at the top level contractor layer. the procurement rules require really only huge companies can bid, which means they can act in a semi-cartel way. this is probably difficult to solve given signalling stuff but in more basic construction projects it feels like not much competition is going on (not suggesting there is a cartel here, fwiw, just feels a bit like it!) 3) on the tube the access is extremely restrictive. you tend not to close the line for extended periods of time (though this is changing recently as i think people are realising the cost of keeping the line isn't worth it vs doing a week closure), which means that you have to get all your equipment down to the tunnel (which might taken an hour), do 2-3 hours of work, then remove all the equipment, in 4-5 hour blocks each night between midnight and 5amish, which means you are paying people often "night working" wages for working really 2 hour shifts, instead of paying them normal wages for working through the day for 8 hour shifts. This probably increases the cost of labour by 5-10x.