At first glance, the Railworks texture tools are very "all or nothing"... you can have this texture or that in a given area, and perhaps with a short changeover area. In fact they can be used to much greater levels of control, but the key is the strength (called Speed) control in the UI, and when used with Wang-type textures.
The thing to remember with the textures is that they are applied from an invisible (central?) point and radiate out from that, with a shape defined by the first parameter ("Shape") and a strength or influence defined by the second ("Speed"). It is possible, though harder, to mix two patterns by having very low strength values, so that they don't "overwhelm" each other. I mostly use a Shape in the of the middle 0..1 range, such as 0.3 or 0.5, and a strength of between 0.02 and 0.05. You also need quite large size values for this to work well: 20-ish is the smallest I would consider, and 35-ish would be more normal. Sometimes, to do this, you have to go too far on one texture and then reapply the other to achieve a good balance.
One last point. RailWorks can use two texture types: "simple" and "wang". Simple textures are just that; a tile that can be replicated over an area. Wang textures are a variable pattern of 8 tiles that can be used to avoid the appearance of a repetitive pattern over an area. I believe that you can only achieve good feathering when using Wang textures: simple ones don't feather at all well. Be aware that in general you can only mix two textures: attempts to mix 3 will cause one of the others to disappear entirely.
I have attached some pictures that hopefully demonstrate this in action. Hover your mouse for the title...