New track?

I assume you are talking about tubular track rather than Fastrack. If so, here are some  ideas. I'm building a layout right now with tubular track, and I've done all of what I've listed below.

1. Make sure there's not any residue from cutting left inside the opening. If there is, a round Swiss file will take it out. It's unusual for factory track to have this, but you never know. 

2. You can spread the opening slightly with a hand-held small tapered punch or nail set.

3. To size the opening perfectly, take an extra steel pin, insert it in the opening and hammer it in with a small (I use an 8 oz.) hammer until it is just past halfway in. Then pull it out with a large pliers and you should have just about a perfect fit. 

For the opposite problem, opening too loose, grasp the flat part below the opening in a narrow gas pliers or a needlenose and work it back and forth slightly. This will overcome the tendency of the metal to spring back when pressure is released. 

Hope this helps. Good luck. 

You don't want to spread the Track. You want "TIGHT" conections. Use a small BRASS hammer to tap the pieces of track together.

FREDSTRAINS

With new track I always insert ONE pin at a time.  Hold two pieces of track with gloves.  Insert one pin into each of the three holes.  Then connect the two pieces together. Really, it's the easiest, best way.

That's one I forgot to mention, and I do it all the time. It works well if the fit is only a little bit too tight, and assuming the pins are properly staked. If the fit is very tight and/or the pins are loose in the piece you're inserting, hammering in an individual pin and pulling it out with a pliers works better. 

Which brings up another point. Everybody who is building a tubular layout of any size should have a track pliers to re-crimp loose pin, and to crimp the pins if you have to custom cut a piece. I really can't think of a way to crimp in the pins without a track pliers. 

Just insert one pin of a 2nd section into each hole consecutively before trying to assemble all 3 at once.  This enlarges the hole just enough without creating slop.

johnsgg1 has the easiest and best method:

Using leather work gloves insert one pin at a time into each opening of each section and then join using gloves and voila: perfect fit each time!

Then when you have the track togehter, crimp the connection with track pliers!