Purpose of straight tracks with titling trains?

I thought the purpose of tilting trains was to eliminate the need for straight tracks on high speed rail lines? Is there only so much a train can do? It's impossible to eliminate all turns on any rail line. I've read something about a minimum curve radius. What is that?

I don't think so. Tilting trains still need straight track to reach their full speeds; but the tilting allows them to save time when passing through curves. Tilting also produces a smoother ride for passengers.

For example, a regular train may be required to pass through a curve at 59mph; but a tilting train can pass through at 79mph. That means the train can maintain a higher mean speed.

This is information on minimum curve radius.

http://en.wikipedia.org/wiki/Minimum_ra ... rve_radius

frischee112 wrote:I thought the purpose of tilting trains was to eliminate the need for straight tracks on high speed rail lines?

Are you under the impression that all high speed trains are tilt trains? They are not. TGVs, ICE trains, AVEs (excluding the Talgo 350) et cetera, do not tilt at all. One other example of a tilt train on a high-speed right of way is the N700-series Shinkansen, but that was built for 8200-foot-radius curves that restrict non-tilting trains to 158 mph (the N700 runs at 168 mph through such curves).

Tilt trains have primarily been designed for operating at (relatively) high speeds on traditional railroads. Examples are the X2000 in Sweden, the ICE-T/TD in Germany, the Pendolino, many examples of Talgo that don't operate on high-speed rights of way (those that have the Pendular system), the stillborn British Rail APT, the Class 221 Super Voyager DEMU, and the Acela Express.

True high-speed railroads have four reasons to minimize curvature, even with tilting trains:

1. Increased safety
   Picture firing a gun along a curved metal plate to redirect the bullet away from a collection of fine china. You'll be fine for a while, but eventually that bullet will wear through the plate and fly straight through your Wedgwood. The gentler the curve, the less friction exerted on the plate (or the rails, wheels and so on) and the less risk one faces.
2. Reduced maintenance cost
   See above - eventually that plate (the outboard rails) will wear out. In addition, tracks have a certain amount of superelevation (banking) in curves, which may range from zero on slow freight lines, to close to a foot of height difference between railheads on high-speed passenger lines. Running a slow freight on a highly-superelevated line will disproportionately wear out the inside rail, and running a fast passenger train on a line with no superelevation will similarly wear the outside.
3. Reduced operational cost
   According to The Railroad: What it is, what it does (4th ed.), each 1 degree of curvature adds 0.8 lb/ton of resistance, or the equivalent of a 0.04% grade. This isn't a lot for a small train like Amtrak runs - maybe 500 lbs-force resistance for a 700 ton train traversing a 1 degree curve - but is a major deal for 10,000 ton freights tackling 5 or 10 degree curves. Reduce the curvature and one reduces the energy needed to pull a train.
4. Increased passenger comfort
   Since tracks aren't generally superelevated correctly for fast passenger trains (see point 2 - most tracks are meant for freight and slower passenger service) Amtrak usually runs at an imbalance over freight tracks, and centripetal force squashes the passengers against the wall. Tilting trains help with this, but a longer-term solution is to ease the curves.

Straightening tracks negates the purpose of/need for tilting trains. That's why most high-speed trains do not tilt.

frischee112 wrote:Is there only so much a (tilting) train can do?

In a word, yes. When the X2000 was tested in the US in the early 1990s, it was famously tested at 25 mph above the speed limit on several curves. Planners told us they weren't even using the "full" tilt so as not to give us unrealistic expectations. Fast forward to today. If you read Amtrak's employee timtetable, Type A trains (tilting enabled Acelas) are allowed 15 additional mph over Type B (tilting disabled Acelas and Amfleet consists) on curves. Moreover, ALL curves (except carefully laid track shifts in Kingston) have an absolute maximum of 130 mph no matter how gentle they are.

I don't have access to in depth information on overseas high speed "heritage" lines vs. new construction. Some of Acela's less than expected performance may have to do with the unusually strict rules of the FRA. But the only way to get sustained high speed running for hundreds of miles is to build new lines from scratch. If it could be done on existing right of way, other countries wouldn't have spent billions on brand new routes. The US will not be economically or politically prepared to do this for some time to come.