Pre-covid wasn't much more. You'd have to go quite a long way further back to get closer to that figure (before TVMs, e-tickets etc). To be making that much you'd have to be taking roughly £1,000 per shift. As
@scrapy says, it might be possible at one of the highest earning depots on permanent earlies, but the vast majority of guards at local TOCs wouldn't get close. Right now I'm in the fairly high in the ranking of revenue-takers at my depot, so you can see that it's not really a significant figure. Don't get me wrong, commission is a nice bonus, but you'd earn significantly more doing a single rest day per month.
Harrogate used to be another, lots of business travellers buying anytime singles to London on train with company cards. Those days are over.