There was a trial of new weedkiller wagons this week. It operated from Derby to Kidderminster SVR on Wednesday 16th June.
"66771 and 66709 in top and tailed formation with the a new weedkiller set KFA VTG 95380 95379 95378 are working 6Z59 10:07 Derby R.T.C - Kidderminster SVR."
I'm not sure if they returned to Derby or now trialling on SVR.
There is a press article here from earlier this year https://www.gbrailfreight.com/gbrf-unveil-partnership-to-transform-railways-weed-control-maintenance/?fbclid=IwAR2MwXb7nG-sfWFvF8u1mqi5i2OMHNYErPDbwF3rpojMVs9m1XRMDBwCKtQ