Master Keys and T Keys and BR1B Keys are extremely easy to get hold of. They appear on sites like Ebay and Gumtree every month. They are sometimes sold at stalls at heritage railways and model railway events and things like that. BR1B Keys are even available brand new from China from dozens of sellers online.
I think the issue is who is possesing them and what they do with them rather than whether or not they possess them. I know of many enthusiasts who have railway keys simply for collection purpose. Many of them collect railway keys amongst other railway stuff but leave them at home in their collection. So if you have these railway keys simply for collection purposes at home than there is zero issue with that. They are not restricted items and there is no law against owning or selling or buying these.
I used to work on the railways as a Guard from the 70s to the 90s and i still have about twenty keys from my time on the railways. I also have a lot of newer keys from modern trains that i have got through friends who work on the railways. But all of these are just for collection and are kept at home amongst all of my other railway stuff that i collect.
The issue is the idiots that use keys to get in to cabs and blow horns and cause all kinds of trouble.
This video is a good example of why the wrong people should not get access to railway keys:
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The video shows two groups of enthusiasts in opposite middle cabs blowing horns at each other and acting like idiots. In another one of that persons Tik Tok videos you can see they have a bunch of railway keys which explains how they accessed the cab. One of them also stupidly sticks his head out the window and gets hit by some bushes. He is lucky it was not going under a bridge or it could have been a repeat of the Wandsworth Common 442 decapitation that happened about eight years ago and he would be dead.
Ideally the railways should introduce a more secure type key. Master Keys and T Keys and BR1B Keys are not secure at all. Thousands of people have them and anyone can get their hands on one. T Key locks are the worst because they can even be opened with a flat piece of metal or some cardboard so you do not even need a T Key for these locks.
One good example a much more secure key is the Aventra/Electrostar door panel key. I have not known of any of the wrong people to get one of these. This key is available online but almost no enthusiasts have one. It does not have a commonly known name and you have to know the key code to find it online. I will not say what it is called but if you know the full number than you find it and can order it online easily. Most of the Guards who use this key everyday do not even know what the official name is as it is just referred to as an "Aventra Key" or "Electrostar Key" or "375 Key" or "377 Key" or "701 Key" or "730 Key" etc by most staff. It also looks like a normal house key. I believe that this Aventra/Electrostar door panel key has been in use for over twenty years now so it goes to show that having a more secure train key is possible if you do not give it a commonly known name and make it look like a normal house key.
I know all the new CAF stock also has a more secure door panel key (like the Aventra/Electrostar key it is impossible to find it without knowing the exaxt name and code) and i have never seen any enthusiasts or the wrong people get their hands on these either.
Also the RKB221 / RKB221A / RKB222 / RKB222A keys are very secure too as these seem to be very well controlled despite being around for decades and you can not order these online without being a railway company.
So this goes to show that having a more secure key is certainly possible if the TOCs try hard enough like the above examples.
But for some reason for cab doors and lots of other stuff we seem to be sticking with T Keys or BR1B Keys even on new modern trains and this is the exact sort of lock that should have a far more secure key.
Regarding the drivers Master Keys this key has been used by almost every train since BR days. Almost all of the diesel locomotives and electric locomotives and all modern DMUs and EMUs use this. I think the 59 66 67 69 70 (which use the even easier to get hold of North American reverser key) and the 68 88 93 99 (which use a much more secure Mainland European style key) are our only trains to use a different key. Even most trains in Australia and New Zealand use the same drivers Master Key that we use. So we seem to be very reluctant to get rid of this and use something else.
I think the main priority should be changing cab door locks to something much more secure (not a T Key or a BR1B Key either) as this is the main thing being accessed by so many of the wrong people judging by Tik Tok videos.