These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
In the original game, building a long enough tunnel would cause the costto run negative, and you would make over a million dollars/pounds/whatever. Bad in that if you earned enough money, your cash would flip into the negative, and you would suddenly be millions in debt. Fixed in subsequent versions.
In the original, with level crossings, whoever owned the railway owned the square. This meant that you could screw over AI road construction by laying rails over their roads and destroying the road. Fixed in subsequent versions.
The original game included two minor glitches in the way it worked out signalling. A railway depot counted as a normal piece of track for signals, and the signalling algorithms didn't distinguish between the tracks of different companies. On their own, these bugs had no real effect. Together, however, you could build a loop of track between the end of a rival's railway station and the nearest train depot. Any train that entered the station would then trigger its own signals, and it would never be able to escape the station.