The main cause (often somewhat hidden behind the term "decayed infrastructure") is that there are too many trains on too few tracks. There are many reasons for that. I think the main ones are:
* Political pressure to have more trains, without an adequate increase in infrastructure capacity (trains are cheaper than tracks and can be delivered faster). For example, political pressure utterly destroyed the reliability of the local rail system in our area, because the number of trains per hour was increased by a factor of 2-3, with only a minimal amount of new tracks (the majority of the network is still single-track). Apparently, the system worked in simulations under near-perfect conditions (no delays, few passengers, no technical problems). So let's build it! The chaos that ensued during the first few months after the network opened again made national headlines. Another example: the highly overloaded Rhine valley line between Mannheim and Basel was proposed to be upgraded to 4 tracks in 1964. In 1970, the project was scheduled to be finished in 1985. Currently the (ambitious) goal is to finish the project in 2041 [0]. The original line (270 km) was finished after 17 years in the 1840ies.
* On regional and local lines, a tendency to increase train frequency and to decrease train capacity (more trains, but shorter ones). I suspect this is also because of political pressure ("your station now has 4 trains per hour!!"), but it doesn't make any sense. A short train which can hold 150 passengers occupies exactly the same amount of "space" (blocks) on the tracks as a full-length train with a capacity of 1,200 passengers, and they require exactly the same amount of personnel.
* Privatization of DB on the early 90ies, with political pressure to be profitable. Tracks are expensive to maintain, so those parts of the infrastructure that could be classified as "redundant" were dismantled. Now they have a network with little redundancy, which is great from a short-sighted business standpoint, but terrible for reliability.
[0] https://en.wikipedia.org/wiki/Karlsruhe%E2%80%93Basel_high-s...
Do you have a citation for this?
The people I know from the DB bubble are telling me that while some places have not enough track (e.g. the infamous Frankfurt-Mannheim/Riedbahn), but the everywhere problem is that there's just fault over fault over fault in tracks (often switches, but even tracks themselves) and trackside equipment.
> […] the number of trains per hour was increased by a factor of 2-3 […]
Even this I'm not quite willing to accept without citation; the railway timetables in the 70ies and 80ies, especially after the oil shock, were quite dense.
I don't, this is just my personal experience with passenger rail in southwestern Germany. The smaller lines with 1-2 trains per hour are usually extremely reliable, while the lines on crowded tracks are usually delayed, or cancelled completely. Note that many of the reliable lines I regularly used over the past 10-15 years ran on track equipment from the 19th century, some still with wing signals and switches operated via pulleys. (Anecdotally, I never experienced any technical problems with this old equipment as a passenger, the technical problems usually started after modernization).
> Even this I'm not quite willing to accept without citation; the railway timetables in the 70ies and 80ies, especially after the oil shock, were quite dense.
The factor of 2-3 was for my local network (which was converted to an S-Bahn network a few years ago, and a 30 minute frequency was introduced, with 15 minute frequency during peak hours). Some parts of that network only had 1-2 trains per day from the 70ies to the mid-90ies.