The idea that pace of discovery slowed down is an extremely common and really obvious fallacy.

We only know that discovery was important after it gets widely implemented, what happens decades after invention. Yet, we count it as happening not at implementation time, but at invention time. So recent discoveries that will be implemented in the future are not counted at all, artificially lowering our discovery importance counts.

Also if you use silly measures like railroad tracks per person, or max land mph, you will obviously not see much progress, as large part of the progress is exploring new kinds of activities, not just making old activities more efficient. Any constant criterion like that will underestimate progress.