Jeremy Corbyn described a train he was travelling on as ram-packed. Ram-packed is not in the Oxford English Dictionary, although jam-packed is, and so is ram-jam, which usually appears in the phrase ram-jam full, meaning tightly packed or crammed full. The OED describes ram-jam as 'originally US', although it then goes on to say that 'there is also significant early evidence for northern English regional use', and notes that ram is an intensifying prefix in Scots. The Dictionary of the Scots Language says at ram- 'Used with intensive force before words which gen. imply something forcible, vigorous or disorderly.' As for the etymology of this prefix, the same dictionary says that the etymology is doubtful, but that the word perhaps originates "from the 16th-c. Eng. cant adj. rum, rome, good, excellent, great, but in some cases influenced by ram, the male sheep, connoting blundering, headstrong, violent". Going back to the OED and ram-jam, this dictionary goes on to say that there is evidence of some later use of ram-jam in the Caribbean.
Interestingly, the few instances of ram-packed that I have found on the internet are all examples of Caribbean English. This page, devoted to the US Virgin Islands dialect has 'the club ram pack', meaning 'the club is packed'. In this piece, Francis, a Jamaican, says "I worked in an ultra-liberal research environment (AT&T Bell Labs) that was ram packed with foreigners". The Facebook page of the Trinidad and Tobago Cadet Force (here) has the sentence 'It was a ram packed Church', and this 2013 article in The Trinidad & Tobago Guardian quotes Usain Bolt as saying 'But when I got out there and I saw that it was ram-packed and the energy was still like the Olympics, it was just wonderful'.