The minister for transport at the time, Ernest Marples, had a bias for road, it happens that he had a tarmac business.
It was actually a construction business, which built among other things power stations, ports and dams - they were more than a road builder.
The accusation levelled at Marples was he brought in Beeching with a mandate to run down the railways so roads could be built. That's stretching credibility quite alot - what Marples did do was abolish the BTC because it was in financial trouble, which in turn paved the way for the establishment of the British Railways Board. Arguably that gave the railways more autonomy than they'd had since nationalisation in 1948.
And Beeching was recommended to Marples by Frank Ewart Smith https://en.wikipedia.org/wiki/Frank_Ewart_Smith when Ewart Smith had been approached to sit on the Stedeford Committee which had been tasked with dealing with the BTC's dire financial problems. Beeching had worked for Ewart Smith, who rated his ability to analyse and solve problems highly. It's worth remembering that in 1960 the BTC had accumulated a £ 350m deficit ( £ 6,725 m in 2021 prices assuming only inflation) with a further £ 80m deficit (£ 1,543m in 2021 prices assuming only inflation).
Beeching's proposals were about making the rail network financially stable - the rate of increase being shown there was unsustainable and changes would have been needed whoever was in power.
If you want a genuine example of a politician abusing his position for personal gain, I suggest you read up about T Dan Smith.