The problem with promoting biodiesel from waste oil is that there just isn't enough of it. From reports such as
Climate change: Growing doubts over chip fat biofuel - BBC News we're having to import oil to cope with demand, and only a tiny fraction of current usage is biodiesel.
There's no way you'd be able to shift the majority of diesel transport over to biodiesel without having to use crops grown for the purpose, which leads us back to the problems with those crops competing with agriculture for food.
Would there be enough to replace all current diesel usage? No
But, buses are going electric, small trucks are going electric.
Coventry are going for 100% electric buses, 297 electric buses by end of 2025, so that reduces the amount of diesel to be replaced. We've already got 19 in Birmingham, & NX aren't buying any more diesel buses.
(Yes, the old diesel fleet will probably end up with secondary operators, but they would be replacing even older, less efficient, buses, so would still see a reduction in diesel usage)
As more rail routes are electrified, then the total amount of diesel required reduces anyway, which also reduces the amount of bio required.
There's the currently timetabled for 2030 ban on new petrol & diesel cars.
New diesel car registrations started falling rapidly in 2016/17, & we're now probably back to 2001 levels of 500k a year. It will continue to fall.
Diesel vehicles pre-Euro6 (2015) are charged in some of the CAZ schemes, whereas petrol only has to be Euro4 (2006), & there will be more CAZ schemes coming through. So people will either use those vehicles less, or replace them with more efficient vehicles.
Would we be trying to replace all current diesel used with biodiesel? No
But rail has no choice but to find ways to reduce emissions from the diesel stock still further, & one way is to use biofuels.
Lets use Birmingham as an example.
Buses are already Euro6 compliant, there's also hybrids & electric buses.
The CAZ charges all vehicles that aren't compliant, even private ones, so any vehicles that aren't Euro6 diesel won't be around for much longer.
The standard is already 5 years old, so lets look 5 years into the future.
2026;
The few diesel buses left are 10+ years old, so are due for replacement.
Taxis & private hire are either 100% EV or plug-in hybrid.
Local van fleets are 50% EV.
New St station still has diesel running through it, from XC, TfW & WMT
Snow Hill lines are still diesel.
2031;
Buses are all zero emission
Taxis/private hire are 100% EV
Local van fleets are 100% EV
What's happening with diesel through New St?
What about Snow hill lines?
Even
if they are doing final works to make them 100% electric, what is happening for diesel emissions between now & then?
With all other forms of transport not only doing something about emissions over that time, but also seen to be, rail has to do the same.