A pure cargo rail line is not going to be much quicker or cheaper than HS2 to build, much of the cost is pretty much the same as when you are looking at saving maybe a couple of percent of the cost by doing the lines for "old" speeds vs current/near future ones, you don't put in a standard that is not up to date on new construction for a project that is meant to be in use for decades, not when retrofitting to update it will cost a significant fraction of the total original project cost in in a few years.
Much of the time spend on building HS2 has been down to things like planning, and checking the route for suitability for any form of rail, getting it a bit smoother etc for higher speed trains is nothing compared to that.
In computer terms, you're wiring up your house or office for a network.
You've got to drill the holes, run the cables, do the ducting etc regardless of what speed you aim to use. You're an idiot if you decide that you're going to save a few percent of the total cost by running say cat5 (not 5e, let alone 6), and fit 100mbit switches, although at least with slow switches they're relatively simple to just swap out even if it's still a waste of money to buy obsolete gear*
(unlike major infrastructure where it's extremely expensive and usually very time consuming to make even minor updates).
*I remember working that out back in the 90's when I realised the difference between a hub and a switch, and went onto Jungle.com and picked up an 8 port 10/100 switch with 4 10/100 cards for only about £20 more than a hub cost
(the cards actually made it cheaper as at the time realtek 10/100 cards were around a tenner each).