The Malvern Hills are the highest point of a ridge formed when a deep igneous intrusion forced an edge of a stratum of extremely ancient, extremely hard rock upwards through the later overlying sedimentary deposits. So if you look inside them you find a variety of steeply-dipping strata with an extremely hard and resistant core sticking up in the middle surrounded by different kinds of, basically, crud. Accordingly, when the Worcester and Hereford Railway tried to dig a hole all the way through from one side to the other, it turned out to be one of those tunnels. They were going great guns to begin with and making record rates of rapid progress with the easy digging as they went through the crud, but then they hit the core and it was a case of "oh, bollocks". Now their rates of progress were making records for how slow they were, going down to a few inches per week. I reckon that must have been an average over several weeks because you don't use blasting charges in a three-inch-deep hole, so it would have been something like a month or two drilling the holes one grain of dust at a time followed by a big bang and then they report the average rate based on how many feet of rock the blast got out divided by how many weeks it took them before they could fire it. At one point they tried some early kind of powered rock drill, IIRC powered by steam so on top of not actually being much good as a drill in any case it filled the bore with hot fog. They reckoned it wasn't worth the hassle and went back to doing it by hand. It took them ages.
As well as being ridiculously hard, the core is also highly faulted, so even though it was really difficult to make a hole through it, it was also really difficult to stop it trying to fill itself up again. The crumbly rubbish on either side caused the same kind of problem, probably less unexpectedly. So there was also yet another repeat performance of the same Ruth Falzine show that had played in countless tunnels through dodgy rock and would play in many more, including Contractor Skimping On The Lining for an encore.
So they built the railway up to the ends, but the opening was then delayed by some years while they finished digging the hole for the final bit to go through. They did also have to re-do the river bridge in Worcester because the inspector found it was too wobbly as first constructed, but I'm pretty sure they still got that done before the tunnel was completed.
It is interesting to wonder what might have happened if they had managed to open when they thought they would. That would have given the Midland's part of the ownership of the West Midlands Railway, which included the Newport, Abergavenny and Hereford, the additional backing of an actual connection to the rest of their system at an earlier date than was actually the case. There was at one point the prospect of the Midland taking it over completely, which didn't end up happening, but the line was originally supposed to open before that opportunity arose and in actuality it didn't. If it had done, the Midland would have had more of the relevant influence at the important moment, and the result might have gone the other way.
It would also have given the Midland a connection to Abergavenny before the Heads of the Valleys line started to happen. That was another of the innumerable lines that ran out of money before they had got going and had to pimp themselves round to the bigger companies in the area. It was expected that it would end up with the GWR/Midland interests, but as it happened the LNWR sneaked in the back door and pulled the rug out from under their feet. Again, had the Midland been a significant influence in the area while it was under construction, they would probably have been in with a better chance and all the Valley lines that ended up being LNWR operations could have ended up with the Midland instead. Now that would have been fun.
The tunnel continued to be a problem after it had opened. As well as being unpopular with crews due to being on a 1 in 80 gradient and having an unusually narrow bore, it kept needing repairs as the dodgy rock and dodgy lining did what dodgy rock and dodgy linings do, including actually falling in once or twice. By 1926 the GWR were fed up with it and dug a replacement to the south of the original, with a normal size bore and the gradient reduced to 1 in 90. Advances in plant made it much less of a performance than the original had been, although the geology altered enough even in the few metres between the two bores that it still gave them the odd surprise. They also built it properly so it hasn't kept falling in like the old one did.
They had some interest in the idea of keeping the old one open for the up line (which goes down) and leaving only Ledbury tunnel as the only single-track section between Worcester and Hereford, but they decided not to. This meant that in WW2 the military could use it to keep torpedoes in, with a narrow-gauge tramway to get them in and out. Later on it fell in again and now to get from one end to the other involves crawling through a pipe.
There is only one ventilation shaft because the hill rises too steeply above the tunnel to find a sensible site for any others, and that one is so close to the west end that there doesn't seem a lot of point having it at all. When they dug the new tunnel they didn't bother digging a new shaft, they just dug a gallery sideways from the new tunnel to the old shaft. There is some utterly barmy conspiracy theory that claims the shaft is in the wrong place and has right-angle bends in it for some reason which is both sinister and so silly that I have no memory whatsoever of what it's supposed to be beyond some vague notion of the government putting werewolves in it or something, but of course plain old CBA is the only explanation that's actually needed.
I have no actual idea, but I have a suspicion that there may have been some kind of problem with Worcester tunnel as well. This is a slightly silly tunnel which is only about 200m long. It would have made more sense for the railway just to go round the end of the hill like the canal does, which is only perhaps 1km further round, but in between the two transport projects they had built a load of nobby houses all along the side of the hill looking over the canal, so the railway had to tunnel through behind the back of them.
The approach to the town end of the tunnel is through a very deep and steep-sided cutting, and only about half the length of the tunnel is actually going through the hill. (They would probably just have made a cutting all the way through if there hadn't already been houses on top.) The other half of the tunnel is at the bottom of the terminal 100m of the cutting, with only a thin blanket of fill on top. This makes me suspect that they found they'd overdone it with the slope of the sides of the cutting, and had to build an extra length onto the end of the tunnel to keep the sides of the cutting off the track.