72 watt power supply adapter

These might help, handy to have around. Barrel Jack Adaptor

These were just the first ones that came up to give you an idea of what they are. You can search further and find different quantities, male/female pairs, etc. They are also available from different suppliers and other websites if you look around.

I think RTR12 has your solution.  The issue of current-handling-capability of these adapters has come up in other threads.  From what I can tell, the 72 Watts = 18V x 4 Amps.  I came across this Velleman adapter that coincidentally says 4 Amps!  You might go through dozens of eBay listings for these adapters and never see a current-handling specification.  OTOH it could be there's one manufacturer cranking these things out with everyone else just putting their name on it...

Here's the 2.5mm inside-diameter version from Velleman.  Also 4 Amps:

Thanks for the size correction, I don't have anything Lionel with barrel jacks.

Should this be a female connector?  The 72 watt power supply looks like it is a male and will need a female.

Orkybrown

The terminology of adapters can be confusing.  Often times the adapter is described as what it converts from and to...rather than describing the connector on the adapter itself.  I guess the safest option is to look at the picture.  I'm pretty sure this is what you're trying to do:

yes I think that is what I want to do

Orkybrown