I have a standard sleeper, cut down from its original 2400m length to 1800m, so it's current dimensions are L: 1800m, W: 250m, D: 125m.

I want to make a curved cut through the shortest dimension (125mm), but keep the 'waste' - so it needs to be a smooth cut, as I want to keep both the arch and the piece that's cut out. Here's a visual:

In terms of power tools I only have a small hand jigsaw (max cutting depth is about 80mm), and a mitre saw (which is obviously no good).

There is a local timber yard who have a workshop with various tools, but I'm not an adept woodworker so I don't know if any of them are appropriate; or how feasible this task is with anything else.

Is there a smart way to make this cut?

I have a standard sleeper, cut down from its original 2400mm length to 1800mm, so it's current dimensions are L: 1800mm, W: 250mm, D: 125mm.

I want to make a curved cut through the shortest dimension (125mm), but keep the 'waste' - so it needs to be a smooth cut, as I want to keep both the arch and the piece that's cut out. Here's a visual:

In terms of power tools I only have a small hand jigsaw (max cutting depth is about 80mm), and a mitre saw (which is obviously no good).

There is a local timber yard who have a workshop with various tools, but I'm not an adept woodworker, so I don't know if any of the tools are appropriate; or how feasible this task is with anything else.

Is there a smart way to make this cut?