The larger faster liners of the great(er) nations were often provided with government subsidies so they could be used as Armed Merchant Cruisers in time of war. So they were built with strengthened parts where 6 inch guns could be quickly added. This happened in both the first and second world wars. In the second war the vulnerability of these ships was soon made clear and they were then used as troopships and for other purposes.

The American liner 'United States' was built with a large subsidy towards the end of WW2 so that it could be used as a very fast troopship.

I haven't worked very hard at all to provide this information!

Les