The issue I have is that on the ECML I do not feel these fences will bring a benefit, compared with the inconvenience they will cause.
As I said in an earlier post, I can fully understand the benefit at places where you have a F-F-S-S configuration, like on the WCML or Midland lines. Here it is a simple case that you can segregate off the two fast lines with minimal inconvenience.
At Knebworth, my understanding (please feel free to correct if I am wrong) is during the daytime there are 2x non-stop trains per hour each direction *booked* to pass through the slow platforms, sometimes more. There are also services booked to stop at the fast platforms, or in many cases trains which use at fast platforms at the signaller's discretion. This happens often at Knebworth, it's not just occasionally during engineering work or disruption.
The disbenefit is that all passengers therefore have to walk rather further to exit the platform, and have a lesser area of platform available to wait for trains, and may have to rush to the fast platform if a platform changes at the last minute. At Knebworth, particularly on the down side, it's not uncommon for a train to simply arrive at the fast platform with no prior information.
Likewise Brookmans Park, New Barnet, Oakleigh Park and New Southgate all have booked non-stop trains on the slow lines, so I can't see what the fences will achieve apart from inconveniencing legitimate users. From my viewpoint I see the fences being portrayed as the answer to all problems, and they're simply not, as apart from the inconvenience they cause, they do not prevent people jumping in front of trains at the platforms which are accessible, nor from bridges or other places where the track can be accessed.
There comes a time when we have to accept a certain level of risk - the only risk-free railway comes when there are no passengers, no staff and no train movements!