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Stanley, your explanation does not address the engineer's concern.  Read again, below.

Actually it does.

If the prescribed curvature was indeed as represented, the central stations at Rugby or Warwick ought to be close upon three miles higher than a chord drawn from the two extremities.  If such was the case there is not a driver or stoker within the Kingdom that would be found to take charge of the train.

The first sentence is true. A chord (line) drawn between two points on a circle will be inside the circle.

However, the second sentence does not follow.

The force of gravity increases with decreasing distance  from the center of gravity. Gravity is lower at higher altitudes and higher at lower altitudes.

But every point on a circle is the same distance from the center of the circle. If the earth were exactly a sphere, then every point on earth would be the same distance from the center of the earth and experience the same magnitude of gravity though the direction would be toward the center of the earth.

Thus the engineer's concern is mistaken. All points at the same distance from the center of the earth are at the same gravitational "level". There is no grade up or down.

However, if you did have a straight line tunnel (a chord) running between two distant cities, points in that tunnel would be closer to the center. Travelling that tunnel would involve a grade down to the midpoint, and then a grade up, as far as gravity is concerned.

Actually it does.

The first sentence is true. A chord (line) drawn between two points on a circle will be inside the circle.

However, the second sentence does not follow.

The force of gravity increases with decreasing distance  from the center of gravity. Gravity is lower at higher altitudes and higher at lower altitudes.

But every point on a circle is the same distance from the center of the circle. If the earth were exactly a sphere, then every point on earth would be the same distance from the center of the earth and experience the same magnitude of gravity though the direction would be toward the center of the earth.

Thus the engineer's concern is mistaken. All points at the same distance from the center of the earth are at the same gravitational "level". There is no grade up or down.

However, if you did have a straight line tunnel (a chord) running between two distant cities, points in that tunnel would be closer to the center. Travelling that tunnel would involve a grade down to the midpoint, and then a grade up, as far as gravity is concerned.

Gravitational level? Do you guys just make this stuff up? This premise is an insult to rational thinking because a train travelling for extended distances (on the actual level) would, given a ball earth, be "grade down" in the middle as the track cuts through the ball and therefore, the train would not operate without extreme input of power going one way, but also it would need to brake half to compensate going the other.  This problem would only not apply, if the train is going perfectly around the exact "gravitational level" at all times.  Since gravity has never been, nor can it be, measured, there's absolutely no way to build a track that is "gravitationally level" through various terrains. In the real world, the way we actually experience it, after the train gets going on a level track, it takes minimal energy to keep it going over the track and force is only needed to get the train started or stopped.    

You can precisely calculate and measure this force that attracts every mass to every other mass in the universe.

Except that nobody's ever measured any force of gravity between two non-planetary objects.  It was invented precisely to back the heliocentric theory.  What we experience here on earth is most likey due to the fact that the earth is negatively charged and/or the pressure of ether (which is what Tesla held).