Joining Lionel tubular track O-Gauge to O-27

When I mixed "0" and "027 track, I just reduced the opening in the "O" gauge track to "027" pin size. To do this, put an "027" pin in the opening, and grab the webbing under the rail with a needle nose pliers. Grab a bit of the rounded part of the rail along with the web and squeeze hard. Slide the plier in a bit more to grab more of the rounded part and squeeze again. Repeat this a few times, and the pin should fit nicely.
You can put a shim under the 027 ties if you want. Or you can transition the height.

Transition pins were available at one time, not certain whether it was Lionel that offered them, or a third party.

I've simply driven O pins into the O27 rails to stretch them a bit. Use a pair of end-nippers to crimp the O27 rail around the O pins for a good connection.

Of course the transition piece becomes "sacrificial" if you decide to change your layout. Never throw anything away, though. I've used and re-used transition and fitter pieces many times as my layout has evolved.

Lionel made 0 to 027 track adapter pins for the Bascule Bridge which are still available. These allow 027 track to be used for the bridge, which has "0" rails.  
Lionel part number 610-2498-010.

C W Burfle posted:

When I mixed "0" and "027 track, I just reduced the opening in the "O" gauge track to "027" pin size. To do this, put an "027" pin in the opening, and grab the webbing under the rail with a needle nose pliers. Grab a bit of the rounded part of the rail along with the web and squeeze hard. Slide the plier in a bit more to grab more of the rounded part and squeeze again. Repeat this a few times, and the pin should fit nicely.
You can put a shim under the 027 ties if you want. Or you can transition the height.

Transition pins were available at one time, not certain whether it was Lionel that offered them, or a third party.

 

I agree !  Best solution at minimal cost.  In this case, no cost.  I took a pair of lineman's pliers and drilled a hole through the jaws while I held them shut.  This tool worked great for tightening pins in my "O" my "O" gauge track.  A smaller drill size can be used to make an "027" track plier.  I'm sure someone makes these by now.

When I mix "0" and "027 track, I just force the "027" onto the "O" gauge pins.

Although this part is offered, it will be quite a bump at the transition point due to a design flaw:

It really should be offset, like the GarGraves adapter pins.

You can make your own by starting with three "O" pins, and grind one end(to the half way point) of each 3/4 of the way around to match the diameter of the O-27 track pins/openings.

If you instead grind them to the center like the above Lionel pins, there will be a significant "bump" at the transition.

For an example of this "offset" method, look closely at the GarGraves "O" & O-27 adapter pins: