Like he said, frogs had always been a solid "X" chunk of metal, and it was the switch points that moved. I would like to see a picture or a diagram of a moveable frog switch, though.
Ask and you shall recieve. Here is the MPF as part of a high speed(?) crossover at CP-53 on the Hudson Line
http://acm.jhu.edu/~sthurmovik/Railpics ... -HSTs.html
MPF' are necessary on HST's (high speed turnouts) because the angle is so acute that the resulting gap would not only cause a rought ride and increased maintainence, but also pose a derailment risk. Amt is installing MPF's on many of its limited speed turnouts now because of their smoother ride (a 4-track complete crossover can really throw someone carrying fude back to their seat from the cafe car around) and decreased long run maintainence burden. Many of the NEC interlockings really got into a sorry state resulting in passenger discomfort. MPF turnouts will stay smoother, longer.
An HST is usually defined as 60mph and over. 80 is the most popular speed in this country as it works well with existing signaling systems.
"High speed interlocking" refers to the speed through switches on a diverging route.
Not always. Before the use of MPF's, equilateral turnouts were very popular for high speed switching. On an equilateral turnout there is no "diverging" route because both routes have the same speed requirement, usually whatever the linespeed is and if not the restriction is listed in the timetable. Back in the day, the EL installed 70mph fixed frog equilateral turnouts when it changed its Chicago main to a single track with passing sidings. CSX was always a big fan of equilaterals and the ends of double track main line segments. Near be in baltimore there is an 50mph equilaterial at CLIFTON PARK. Amtrak has one on the Empire Connector at INWOOD. Conrail was never a fan of equilaterals and even if the speed through both routes was 30mph, the "straight" would give a clear and the "diverging" medium cpear. I suspect, but am not positive that CSX may be changing this. I have a video of CP-BERRY on the NY Short Line that appears to show a train getting a straight CLEAR off the passing siding.
Here's a pic of the equilateral at CLIFTON PARK.
http://acm.jhu.edu/~sthurmovik/Railpics ... RK-Int.jpg
Note the signal lacks a medium speed orbital.
Oh, here's a bit of trivia. Does anyone know why its called a frog?
PS: I'll see what I can do about getting those pix up tonight.