Variables
Steaming may work well, but a lot depends upon variables you have not mentioned. You say 125 degrees, but not how long the piece is, nor how sharp the bend (what the radius) is. Will your rubrail fit into a steambox that you already have, or will you have to make one? Wrapping a piece of timber in towels and soaking them with (a lot of ) boiling water may be enough for the bend you need. Boiling the piece in water can also be a little safer than dealing with the scalding propensities of steam. There are rules of thumb for working with different types of wood, and different thicknesses of them. If you're using a form, be aware of "backspring" : when you remove the piece from the form it springs back a little straighter, and may crack when you go to install it. The form needs to "overbend" the piece to compensate. A good place to perhaps get more well-informed information is the woodenboat forum: http://woodenboat-ubb.com/cgi-bin/UBB/ultimatebb.cgi . Steam-bending is certainly a lot spiffier than notching saw kerfs on the inside edge, tthough.