Let's bust this myth with a quick look at the physics and specifications of HSR track alignment.
Discomfort arises when there is a curve in the tracks, because passengers conform to Newton's first law: they tend to keep going straight. The centripetal force imparted by the train causes them to follow the curve, and sometimes to spill their drink. In horizontal curves (left or right), engineers can use superelevation, a simple trick that harnesses gravity to provide the centripetal force and smooth the ride. Vertical curves (up and down) are a different matter: passengers must bear the full brunt of the centripetal force.
The centripetal force is perceived as a vertical acceleration, such as you might feel when riding an elevator, and goes as velocity squared divided by radius.
The recently published HSR design standards (specifically, TM 2.1.2 section 184.108.40.206 and TM 1.1.6 section 6.1.7, which trace to the AREMA manual, Chapter 5, Part 3.6) describe the design limits placed on vertical acceleration: typically just 2 to 3 percent of gravity, obviously much less than would ever be experienced on a roller coaster. Given this acceleration limit and the planned operating speed of 200 km/h (125 mph), the references above contain the following constraints:
|Passenger-Only 125 mph Vertical Curve Length*||840 ft||650 ft||420 ft|
|Freight 75 mph Vertical Curve Length* (sag)||2000 ft||1200 ft||1200 ft|
|Freight 75 mph Vertical Curve Length* (crest)||1500 ft||1200 ft||1200 ft|
| Passenger-Only Maximum Grade||1.0 %||1.7 %||3.0 %|
| Freight Maximum Grade||1.0 %||1.0 %||2.0 %|
What do these mean? To find out, it's helpful to look at a picture of what a vertical track profile looks like when you need to transition from one elevation (or depth) to another. The figure at right shows the basic anatomy of a vertical transition. Passenger discomfort (if any) occurs only in the curved portions at the beginning and end of the transition; the straight ramp in between is not perceived as having any less comfort than flat and straight track. Simply lengthening this ramp will increase the overall height of the transition. Assembling the above specifications into actual ramp lengths, we can calculate a useful metric: the total length of a vertical transition, depending on how much rise is required.
|Transition Rise||Train Type||Desired||Limit||Exceptional|
|15 ft (Ground Level to Elevated)||Passenger Only||2340 ft||2140 ft||1830 ft|
|Passenger + Freight||2830 ft||2600 ft||2600 ft|
|30 ft (Trench to Ground Level)||Passenger Only||3840 ft||3300 ft||2750 ft|
|Passenger + Freight||4730 ft||4200 ft||4170 ft|
|45 ft (Trench to Elevated)||Passenger Only||5340 ft||4200 ft||3470 ft|
|Passenger + Freight||6250 ft||5700 ft||5370 ft|
|90 ft (Tunnel to Ground Level)||Passenger Only||9840 ft||6850 ft||5100 ft|
|Passenger + Freight||10750 ft||10200 ft||8100 ft|
|105 ft (Tunnel to Elevated)||Passenger Only||11340 ft||7740 ft||5600 ft|
|Passenger + Freight||12250 ft||11700 ft||8850 ft|
Keep in mind that even the "exceptional" values would not be anywhere close to a roller coaster ride: the acceleration would be only about 4% of gravity. Roller coasters routinely exceed 100% of gravity.
What immediately jumps out from the table is that the trickle of peninsula freight trains make these transitions much longer and potentially much more community-disruptive than passenger-only infrastructure.
As for Gina Papan's notion of Burlingame getting a tunnel as a consequence of a hypothetical underground Millbrae HSR station: it would take less than a mile for tracks to rise up to an elevated structure that clears Broadway in Burlingame, with the utmost passenger comfort.