Perhaps someone could develop a business case for a Calais to Holyhead sealed-train freight service, using pocket wagons so the containers physically can't be opened on route.
Over the last forty-odd years I have spent many hours sitting on the quaysides of various of the short sea crossing ports. Most intra-european traffic is not containerised - it travels by truck. Most of the traffic through Dover is made up of articulated lorries - very few containers come by roll-on, roll-off ferries. This putative train would have to be a rolling motorway carrying the complete HGV for it to have any chance of carrying any worthwhile volumes of traffic
Or it could work the other way. Politically iffy to be rushing EU-Eire traffic through whilst our own got stuck. And it could be used as leverage if the EU are awkward at the ports.
See above and Dr Hoo's post
above. Even between Ireland and the continent the standard method of transport is the lorry. If this traffic is held up in the UK for any reason then, apart from any immediate legal action, shippers will hire roll-on, roll-off ferries to travel between Ireland and one or more ports on the continent direct - both ends will be in the EU. I reckon it might take a week to set up.
The fundamental misunderstanding about the effect of Brexit in this thread is that any delays/customs clearance/food safety issues will only affect HGV traffic - as that is how the bulk of manufactured goods and foodstuffs are transported within Europe, including the UK. Containers are for deep-sea shipments - pullovers from China for Primark - and by definition originate from outside the EU and are thus not directly affected by Brexit except for the tariffs which may be changed at the port of entry.
The difficulties will start when France starts checking refrigerated lorries full of Welsh lamb for food safety in, for example, the large new area that has just been built at the port of Dunkirk. A delay for each truck for a day or so for the paperwork to catch up will put up the costs of exporting the meat and so reduce the market.