IF your Carriage is able to support it own weight, then all you need to do is move what is there.. In that case a Screw Driven Nut is all you need,

However, it your Carriage does not move, and you need it to, then Old School Thompson Ball Shafting, and Linear Recirculating Ball Bushings will work.. You must also add the Screw and Nut..

https://www.motionindustries.com/products/search;q=Linear%20Ball%20Bushings;origin=search

https://www.motionindustries.com/products/search;q=Ball%20Shafting%20Guide%20Supports;origin=search

Gets Pricey.. But Durability will last a lifetime..

Cap

Could you use these types of rails instead?

https://www.automationdirect.com/adc/shopping/catalog/power_transmission_(mechanical)/linear_shafts/hard-anodized_aluminum_drylin_r/a-awui-12-1000

Then just stack them next to each other for as long as you need. This also solves your support issue since they support themselves the entire way.

Or use these types of rails and carriages instead since you can stack these rail types end to end as well:

https://www.automationdirect.com/adc/shopping/catalog/power_transmission_(mechanical)/linear_bearings_-a-_rails/linear_rails#sort=undefined%20asc&start=0

Or even just get a long piece of T slot and use T slot linear bearings:

https://www.mcmaster.com/t-slots/t-slotted-framing-linear-bearings-and-track-rollers-5/

The T slot rail will either need to be " double" in the vertical direction or have support under it. A single size rail with nothing under it keeping the rail straight will sag. Single T slot rails are pretty weak and bendy over more than a couple feet without something securing it.

"I only see linear rails up to one meter (39.3"). Can they be connected end to end? Is there better alternative?"

You are probably looking in the wrong places, however, a lot depends on your budget. For a simple example; McMaster lists rails up to 3 meters long, for 42 cents a millimeter, $1260 per rail, and one generally wants two plus some carriages (at about $170 ea.)

If one went directly to a rail manufacture I suspect even longer lengths are available, at least in larger size$.

For extremely long lengths they "can" be connected end to end, but one wouldn't want to do that for 84" which is readily available in one piece. a "better" alternative depends entirely on the application and definition of better. If your budget is say, less than $100, one might try skate wheels on angle iron or steel tube, but see next paragraph if you can manage twice the $.

Looking for a cheaper albeit much less reputable source I did a search for linear rail on Amazon and the very first sponsored listing was for a pair of 2200mm (86+ inches) supported round rail for $169.99 apparently form a US warehouse as the say "FREE delivery: Nov 22 - 29"

"GUWANJI SBR20-200mm to SBR20-2200mm Linear Rail with SBR20UU Slide Blocks CNC Parts Linear Rails for Automated Machines, DIY Projects(SBR20-2200)" I'm not sure how to link to the listing

Thanks so much for all the help.

I think the price tag on some of the McMaster shafts litteraly made me forget that they existed as an option.

I'm going to take these suggestions and rethink my approach. Or buy the cheapies from Amazon.

Thanks again,

Brad

Depends on what you are trying to do. In my former job, one of our products was a poly spray system that ran on Thomson shafting up to 240" long, which is roughly 6 Meters ish.

Here are a couple of pictures showing how we did it, many of these are now near 30 years old running in a filthy environment with poor maintenance.

We used L100 timing belts and an Inverter driven AC motor to drive, but the carriage would be the same.