I use ME C55 and C40 flex and I've used ME #6 turnouts too, so when I'm looking at this thread with ALL the advice as to how to get rail joiners on, I'm thinking to myself "Did you ever have problems getting ME rail joiners on ME rail???"
The answer to that question is "No".
First, I didn't ever like the looks of them big 'ol rail joiners ugly-ing up my precision track work, so, I started cutting the rail joiners into much smaller versions that fit between the ties, cutting the plastic between-the-tie-spacers away from at least three ties on either side of the joint then sliding those ties back out of the way of the solder joint. The ends of the rails get dressed with a file so they aren't rough, and the corners of the rail foot are filed at an angle (roughly a 45 deg. angle)...just a bit of it, not clear to the rail web...and the joiners slide on pretty easily. I then bring my second piece of flex to the party, using my Dremel and a fine sanding disk to get the rails just the right length, and after dressing the rail ends just like the other piece's, I slide the shortened rail joiners onto the second piece of flex, centering the joint in the middle of the joiner. I solder it after fluxing with Superior Super Safe #30 gel flux using a tin/silver coreless solder, and a few seconds after it's cooled, slide the ties back into their correct position on either side of the joint.
I don't want my rail joiners loose and flopping around since they are there for me for a structural purpose, not for conducting electricity because I put a feeder on every piece of rail, and they serve this structural function very well after I solder them. ME flex, since it's stiff, doesn't need a lot of structural assistance to stay in place anyway, but a bit of reinforcement with my little rail joiners has worked very well on my portable, sectional layout.
Additionally, since I'm not sliding ties UNDER the joiners, all the ties are the same height, and have their cast-on spikehead/tieplate details intact, just like the rest of the ties on the layout, and the joints virtually disappear...especially after painting, ballasting and weathering.
If I were to use full-length rail joiners, I would welcome their tight fit and consider that to be essential. When working on modules and layouts other than my own, to get ME railjoiners started on ME rail, I first cut one (1) of the little "tongues" off one end of the joiners so they are a bit shorter using a sharp flush-cutter...I'll cut the other tongue off later... then dress the ends of the cut rails with a fine jeweler's file and completely de-burr them. I also make sure I file the small angles on the corners of the rail feet on either pieces of flex I'm joining. This allows me to get the joiners more easily started (the most important part) and easily slid on to the rails.
After puncturing myself once and bleeding all over my unfinished scenery, I LEARNED from this experience and started using one of the corners on my miniature, precision flat-nose pliers to hold the joiner which I've cut ONE little tongue of off on the end where I've left one tongue on, mounting it on the rail with the end where I'd cut the tongue off of. To be perfectly clear, I hold the joiner with the corner of my precision flat-nose pliers on the end I have NOT CUT THE TONGUE OFF OF, sliding the end with the tongue cut off from onto the rail FIRST.
After mounting the joiner on the rail and sliding it on, I cut the remaining little tongue off of the mounted joiner, and slide the other piece of flex (the rails of which I've dressed and filed just like the piece I've mounted the two joiners to) on to the two mounted joiners, then using miniature, long-nosed pliers, I position the mounted joiners where I want them on the rail feet.
This is pretty simple, and it takes a lot less time to do this than to write about it. These joiners won't be flopping around on the rails, and they fulfill one of their intended purposes by accurately aligning the rail ends at the joint, and using my precision flat-nose pliers to mount them, my thumbs and fingers remain unwounded.
However, to repeat myself, on my own layout, I don't use full-length rail joiners, but the ones I cut to fit the space between ties.
I think the full-length ones are overkill, look ugly as sin and necessitate equally ugly, modified ties under them.
Photo (1) - Here's a pic of my cut-down rail joiners mounted to painted, unballasted track:
Edit: I always forget to tell how I cut the joiners. I mount them on a scrap piece of rail, and cut BOTH the joiner and rail at the same time. This prevents the joiner from squashing. I then slide the cut joiner back on to the rail a bit, which bends any burrs on the joiner to the outside of the cut end where I can easily file them off. I can get two small joiners out of one full-sized joiner for sure, and sometimes three.
Cheerio!
Bob Gilmore
