The flatness of California HSR, vs. someday Driftless rail

I recently did a scan through California's Central Valley on Google Maps to trace out the California High-Speed Rail construction zones currently visible there. The imagery there will be lagging the real world significantly, but shows a nearly continuous stretch being built from Shafter, CA outside of Bakersfield up to Fresno. It's mostly just right-of-way and parts of bridges so far, with a few that are structurally complete.

What struck me is how unbelievably flat the area is. The default view of this embedded map uses a "terrain" background layer, and it's hard to find any topographical contour lines (Depending on zoom level, Google appears to mark differences of 40 or 80 feet). You can change the background to an aerial ("satellite") view by clicking the upper-right icon to pop out the sidebar, and then an icon should appear at the bottom of that to use an alternate backdrop.

Someday we'll need true high-speed rail on the Twin Cities–Milwaukee–Chicago corridor, which will involve getting through the Driftless Area of southeastern Minnesota and western Wisconsin. Even getting the Empire Builder / Borealis route upgraded to higher speeds will require dealing with it (those trains are mostly limited to 65 mph between Red Wing and La Crosse / La Crescent today, and historic trains only got to between 70 and 80 mph for most of that segment).

The area is pretty flat compared to much of California, but will be a much bigger topographical challenge than the Central Valley, considering the bluffs were created by the Mississippi River and its various feeders digging a channel that drops more than 600 feet below the surrounding landscape.

It's nothing compared to the roughly 3,400 feet that California HSR will need to climb from Bakersfield to the Tehachapi Pass, or the apparent 2,500-foot drop from Palmdale to Burbank (mostly happening in tunnels under the San Gabriel Mountains). But, the soft karst (dolomite, limestone, and sandstone) geology has made many small valleys that could create the need for lots of smaller bridges and tunnels to run through and between them.

On one hand, we've had some very good experiences tunneling in similar conditions at MSP airport, where the twin 1-mile tunnels apparently only cost about $117 million (probably 2004 dollars, which is still a bit shy of $200 million today if going by consumer inflation), but we've also seen costs skyrocket for Southwest LRT, which has had to deal with much swampier conditions along much of its route.

Eroded bluff faces will probably be more challenging than the flat land the airport tunnels were built through, but hopefully the rock will still be favorable for tunneling through. I'm sure we'll have to create some significant ones if we ever hope to get 90-mph, 110-mph, or faster trains running along the river corridor.