Hi Frank,
From my experiences on working the Claremorris ESSO liner trains in days gone by,
It was always a requirement for those trains to have a barrier wagons front and rear,
The ESSO liner had its own dedicated two axle barrier wagons-one had a 20'orange CIE container on,with the other being just a skeletal flat,
Any type of flat/Guinness/bogie wagon could be used if those dedicated wagons were not available,
On one occasion I remember having a spare 40' 30XXX type bogie wagon behind an 071 loco from Ballina to Claremorris and then used as a barrier for the oil train onwards to Northwall.
That bogie wagon was left on the oil train for a number of weeks before being replaced by the normal two axle wagons.