You're going to "daylight" a tunnel, a common thing now as railroads try to eliminate the cost of tunnel maintenance. The type of cut that's left is determined by the type of rock the tunnel went through. If the tunnel wasn't lined but was drilled through solid, competent rock, the cut would look just like the inside of the tunnel without the top. If the tunnel was lined with timber or rock walls, the lining would usually be demolished, the cut would be widened, and new retaining walls would be built. How the wall would look really depends on your era. Up until about 1920, many unstable cuts were lined with rock or timber retaining walls. After that, they were almost all replaced with either cast concrete walls or interlocking concrete blocks. There are a still a few places with rock retaining walls, which look really cool, but they are very rare in the modern era.