TL;DR – This is an ability to double stack pallets inside of existing intermodal container size profile, shipping less “fresh air”, not two intermodal containers one on top of another in the rail wagon as commonly seen in the US. That is fundamentally not possible on UK railways (the “loading gauge” is much smaller in the UK), or indeed in many parts of Europe (more wide spread use of electric locos powered by high voltage overhead catenary rules this out.)
Even within the UK (small country) it has been cost effective for a while to “trunk” intermodals by rail on certain routes (southern and western ports to centralised warehouses in the middle; middle north to Scottish lowlands); typically a once per day train path in each way chartered by a major retailer (usually Tesco). [Railgeek note: you can spot these moves “in flight” on sites like realtimetrains.co.uk if you know what to look for.]
Also there is a rail trunk for chilled produce from Spain (with wagons that can change track gauge in motion from larger Iberian gauge to standard), again Tesco sponsored.
The East-West “Silk road” rail trunk (bulk and ecommerce parcels) from Western China though Central Europe to e.g. Belgium and exceptionally UK has been ceased because of Russian sanctions.
Is this a case of operators trying to get more inventory in freight containers? Why now?
Yep. In country transport costs (trucking but also trains) within the UK have risen significantly, so cramming more stock in the containers on the train wagons is a good idea in theory; the containers are clearly filling from a floorspace perspective rather than being limited by total cube or tare, and doublestacking means they will cube out. I suspect the complexities around this solution means it will have a long payback period.
On the other hand, this might be a bit of greenwashing by Nestle (I’m undecided whether Nestle is trying to reform as a business or just doing cost reduction/greenwashing)
On a related point on UK roads we commonly see high roof trailers which can double stack roll cages (taller than pallets), on a lower floor that is usually lower than a normal trailer and an upper floor, overall just fitting into UK motorway loading gauge, although with these there is the “goose neck” space over the fifth wheel which is hard to use effectively; the lower floor being lower to the ground than the fifth wheel. UK road vehicle max length specs are a bit shorter than US or parts of Europe, although there are pilots of increasing the length, requiring the trailer to have auto steerable wheels to achieve necessary turn circles for practical use in existing yards/bays and urban areas.
A few retailers (and parcel carrier trunking vehicles) use non-articulated trucks (rarely with demount cargo body) and 4 wheel droppable trailers, but still within same max length that articulated vehicles also have to comply with.
If you want to see videos of UK retail logistics, I can suggest Eddie Stobart: Trucks & Trailers - Wikipedia which now seems to be available on YouTube. Junk reality TV at it’s finest, but with occasional professional interest; at the time Stobart was one of the main trunking hauliers for Tesco (who also operate a significant fleet inhouse, mainly covering warehouse to store, with some supplier backhaul).