The line is mainly used for light engine movements between Warrington Arpley and Halewood or Ditton Foundry Lane.
The best bet for seeing a freight train would be 6O16 (Novelis aluminium to Germany) from Ditton on a Friday. It's not a guaranteed runner but does take that route fairly regularly on a Friday (other days tends to run as 6O26 via Runcorn).
6O16 is seen here taking the low level route via Warrington earlier this year:
https://www.flickr.com/photos/jjm2009/53633377921/
There's also a wagon trip (empty STVA car wagons to or from maintenance) which does Warrington > Halewood > Garston > Halewood > Warrington usually on a Tuesday, running as 6F42 to Garston, then 6F82 to Halewood and 6F49 to Warrington. The problem with that one is that it is often cancelled or runs light engine, and can often run very early back from Halewood. For example yesterday it ran light engine in both directions and just did a bit of shunting at Halewood, and the week before it was cancelled completely...
The 6F49 move is seen back in 2015 in this carefully composed shot with the power station in full effect in the background:
https://www.flickr.com/photos/8arail/16784768459/
Timings can be seen on RealTimetrains at Fiddlers Ferry S.B. - for example this is yesterday when 6F42 and 6F49 ran light engine as 0F42 / 0F49. No guarantee anything shown there will run on any particular day, but at least gives an idea of the timings for when 6O16 / 6F42 / 6F49 might run:
https://www.realtimetrains.co.uk/se...24-08-13/0000-2359?stp=WVS&show=all&order=wtt
Apart from those workings I can't think of any other regular freight that uses the line.