PT lumber is already quite soaked with the water-based preservative that makes it "pressure treated". You will likely be well into retirement before soaking in water increases their flexibility. Some folks use steam on thin strips of wood, but not lumber.
What is the radius of the curve of the bridge?
If it's going to take more than one 8' board, how will the transition be handled?
Some curved boards are created by making multiple partial parallel saw cuts. However, most PT lumber is pine which will likely warp and twist as it dries. I would not build the curve in place on the bridge. Build it against a form laying flat and install it after it has dried a while. That way, if it turns out crooked, just don't install it.

It will take 1 8 ft board, and the radious is about 50".

The circumference is 314.16" so the 96" board is a little more than 1/3 of the way around. Mark your circle out on a sheet of plywood and secure vertical stakes around the perimeter. Dry out the wood and make saw cuts that allow you to bend the board while remaining in contact with the stakes. Obviously you can practice with an inexpensive 2x4 that is already dry to determine how many and how deep each saw cut needs to be. When the actual board is in place and is dry, use glue on the back side to help it retain the proper shape.

I have not tried this myself but I have seen on a few very high end decks that curved wooden handrails are typically laminated, in other words you will need to rip your stock into whatever thickness it will bend at, maybe 1/4 " or 6mm, and then lay them over the forms gluing(waterproof) and clamping as you go. This is exactly the method prescribed to me by the designers of those rails. Using a form with adequate plot points will ensure that you get a radius rather than chords with angular transitions and straight in between.

Where ends meet if the stock isn't long enough a simple scarf joint is made but you never "stack" the scarfs, you overlap them for strength.

tkendr01, I'm afraid that with your prescribed method the handrail will be neither strong nor asthetically appealing.

The backcuts necessary to bend the board will need to be most of the way through to the front surface and spaced fairly close together to provide a smooth radius, do you suggest filling them in with some material after the rail is at it's desired form and in place? I have never seen this method of board bending employed anywhere that it will be seen other than temporarily such as a concrete form. I also don't understand how gluing the backside will help the board retain it's proper shape unless the glue is adhering the rail to something, but over the long run mechanical fasteners will probably be a better choice because the minimal surface area to be glued at the points of attachment will probably break due to moisture related expansion and contraction. This won't be a problem between the laminations because of the large area being glued relative to the thickness.

If you go the lamination route make a form from scrap plywood or particle sawn to the radius you will want.
Now go and look through the 2x12 or 2x10 lumber for the ones that will give you the most straight grain knot free strips and rip your pieces from that portion of the 2x10or 12.

madreipper>>>>It will take 1 8 ft board, and the radious is about 50". I'm not sure what that means ? PT lumber is... kind of weird to work with, it's long lasting if applyed properly. But has lots of draw backs...

I agree that laminations of 2x10 or 2x12 glued together is a much better answer. Using material other than PT pine would be preferable as well.
If someone just must use PT 2x4 material, then I still stand by my recommendation.

Either shop at Home Depot and they all come pre-warped or laminate one using exterior glue.

My dad cut wood for a living until he was 80 something and have seen him cut multiple curfs for quick bends .. he'd prepare that 2x4 to accept side and bottom skins before he cut the curfs and put it all together. You would think it nothing but just a bent 2x4 when done.
I'm sure he would prefer to laminate multiple thin slices. That takes some time though.

I'm a little puzzled in your description of the curve. The 50" radius doesn't seem right to me. I did a layout of an arc of that radius with an 8' arc length and the bridge wouldn't be walkable. If your talking about having the same curve as the bridge with the rail up about 50" from the bridge that makes more sense to me although it is probably high. I'd be glad to help with a jig layout but would ask first to verify dimensions. Go to the side of the bridge and take a straight line dimension from top of tread across the span. Take a straight line dimension from the center of the line up to the top of tread at center of bridge. A three point arc will give the radius of the bridge deck and it would then be easy to add the desired handrail height to get the accurate radius to layout the rail jig. I'm reading between the lines but if the bridge has a 12" rise at center and your want the handrail up 38" from the deck that wouldn't equate to a 50" radius to form the wood. I'm not trying to sound critical but as the actual radius gets larger, the work that you are trying to do should become easier.

I'm sorry - the radius is 100".