What intrigued me in this piece was that there was a rumour that only three vehicles were being considered.
It might help the discussion if I described the current WHL operation.
In winter just six units are used. In summer three additional units are 'borrowed' from the central Scottish fleet, presumably to the annoyance of the commuters whose trains are shorter. These units are added to the Fort William trains.
It is my understanding that the franchise does not allocate any extra units to the route, the 156s will be replaced with the same number of 158s. Each of these units will have 2 reservable bike spaces, plus two non-bookable in the disabled spaces. Thus a fort William train can carry at least four bikes, more likely six to eight. An Oban train can carry two, not more than four.
Everyone knows that this is inadequate, but it is up to Transport Scotland to fund any enhancement.
Although the additional Oban services load well, they have abstracted little traffic from the joint FW/Oban trains. In particular, the oban portion of the 08.21 from QS is often over capacity when it leaves Crianlarich. The record is, I believe, over 100 standing PAX, plus their backpacks and bikes. Therefore the conversion of three 153s to be added to the Oban units would seem a reasonable idea. As has been pointed out, they will be going for scrap at the end of next year, so will be cheap. The conversion will be limited to removal of the toilet, instead of fitting retention tanks, and of a quarter to a third of the seats to provide the bike and luggage storage.
There is however one problem. This will make these Oban/FW trains 7 vehicles long. While QS will be able to take such a train by 2020, the WHL stations from Helensburgh Upper to Crianlarich all have platforms for six vehicles only, as far as I know. (I'm sure that there are posters here who will be able to correct me.) If that is the case, it would be necessary to run separate trains, which would have consequences for staffing etc.