Some facts about MSTS tracks and the prototype
The way we designate track sections in MSTS is more closely related to model railroading terms than to the prototype. Prototype railroads measure curves by degrees while we use radius, switches (turnouts) are measured by numbers while we use degrees. When working from prototype information this can become a problem to know what sections to use. Here I try to explain some of these terms and present charts to translate them to MSTS measurements.
Names
Many find the names of the track sections confusing though once you learn the system there is some logic to it.
First is a prefix description of the type of track ('A'=standard normal gauge, 'N'=XTracks narrow gauge etc). Other add-on track packages usually have their own prefix like 'UKFS' (UK Fine Scale) or 'SR' (ScaleRail). Different track packages usually don't mix very well since the rails look distinctly different. The standard 'A' type track is the only one that mixes well with dynamic track. XTracks 'A' type track is designed to mix with and complement standard track sections. Next is a number followed by the letter 't', this is the number of parallel tracks ('1t'=single line track, '2t'=dual line track). After this it depends of the type of track. There are three basic variations; straights, curves and others (switches etc).
For straights it is the length in meters ('10m') followed by 'Strt' (straight).
One meter is about 3'4" and 1' is about 0.3 meters, a yard is about 0.9 meters.
A ten meter straight is thus roughly 33'4" or little more than eleven yards.
For a curve comes the size of the curve meassured in meter radius ('500r') and degrees ('10d'). The bigger the radius the wider the curvature, a small radius equals a tight curve. The degree is a meassure of the length, a '20d' curve is twice the length of a '10d' curve. A '360d' curve would be a full circle (not very usefull), '180d' a half circle and '90d' a quarter of a circle. If you look to replace two consecutive a1t500r10d curves with a single section you should look for the a1t500r20d, not the a1t1000r10d. Both sections are the same length (twice that of an a1t500r10d) but different curvature. Same radius curves have same curvature which is important to remember. If you want to know the length of the curve meassured along the tracks this can be calculated from the radius and degree (see Curves below).
Other sections have some description of the type of section after the 't' like 'Pnt' for a switch (point) or 'Xover' for a crossing (crossover). There are many different variations like 'Brdg' (bridge sections without track bed), 'EndPnt' (special section for use with switches), 'Frog' (frog module for modular switches), 'Stub' (stub switches) and 'YPnt' (Y switches). New variations are added from time to time but usually it is easy to make ut what it stands for. After this is often some kind of size meassure either by length or radius like for straights and curves. Switches and crossings are usually designated by the degree angle between exiting tracks. The diverging track of an a1tPnt10dLft switch is on a 10 degree angle of the straight track. Note that you would need to know the radius also for a full description of the switch but this is omitted from the name.
a1tPnt10dRgt
a2t1000r5dTun
Some sections (usually switches) finally have a suffix description of their variation like 'Rgt' (right hand switch) or 'LftMnl' (left hand switch manually operated). Other descriptions that can be applied for any type track are 'Tun' (tunnel), 'Wtr' (special water sections), 'Concrete' (alternative track bed). These descriptions can be combined in different variations and as above new descriptors are added from time to time.
The example at the top of this section is a dual line 500 meter radius 10 degree curved track section inside a tunnel. You can find another explanation of track section names at the Steam4me site together with other good tutorials.
Gauges
Prototype railways have used practically every gauge you might think of. In practice though some gauges are more common than others. The most used gauge and what is usually termed normal gauge is 4'8½" or 1.435 meters. Another much used gauge is 3' or gauges close to this or 1 meter. The gauges in MSTS and XTracks are the following (also including spacing for parallell tracks):
| A*t | N*t | UKN*t | |
|---|---|---|---|
| Gauge [feet] | 4'8½" | 3' | 2'4" |
| Gauge [m] | 1.435 | 0.861 | 0.7175 |
| Spacing [m] | 4.985 | 2.991 | 2.4925 |
Dual gauge tracks (D1t) of the DualTracks package is a combination of A1t and N1t tracks in the same shape using one common rail for the two gauges.
Curves
Prototype railroads usually measure curves by degrees (very thight curves might
be specified with radius). This is something completely different than the
curve degree of a MSTS curve. The prototype degree is related to the MSTS
radius of a curve, look at this picture: 
The degree of a prototype curve is defined as the angle for a curve section
with the chord 100 feet long. In MSTS we use radius instead but these two
measurements are related thus:
R = 15 / SIN(D/2). If you are not into mathematics here is a table listing the
common MSTS curves and prototype degrees.
| MSTS R [m]: | 60 | 90 | 120 | 150 | 250 | 500 | 1000 | 1500 | 2000 | 4000 | 8000 | 15000 |
| Prototype D: | 29 | 19.2 | 14.4 | 11.5 | 6.9 | 3.4 | 1.7 | 1.1 | 0.86 | 0.43 | 0.21 | 0.11 |
Please note that the relationship is reverse; the larger MSTS radius curve (R)
the smaller prototype degree (D). For large radius curves the relation is
approximately; R * D = 1700.
Or if you want to calculate radius from prototype track charts; R = 1700 / D.
Sometimes you want to know the length of a curve meassured along the rails. This
can quite simply be calculated from the MSTS radius (R) and degree (d) if you
know a little mathematics. The formula for the length in meters looks like
this: L = R * d * 3.14 / 180.
Here is a table for some of the common curves in MSTS with their length in
meters:
| Radius \ degree | 60 | 90 | 120 | 2501 | 500 | 750 | 1000 | 1500 | 2000 |
|---|---|---|---|---|---|---|---|---|---|
| 1d | 1.047 | 1.571 | 2.094 | 4.320 | 8.727 | 13.09 | 17.45 | 26.18 | 34.91 |
| 5d | 5.236 | 7.854 | 10.47 | 21.60 | 43.63 | 65.45 | 87.27 | 130.9 | 174.5 |
| 10d | 10.47 | 15.71 | 20.94 | 43.20 | 87.27 | 130.9 | 174.5 | 261.8 | 349.1 |
| 20d | 20.94 | 31.42 | 41.89 | 86.39 | 174.5 | 261.8 | 349.1 | 523.6 | 698.1 |
1) The 250r is actually 247.5075 meter radius.
As you can see from the table if you double the radius or degree the length is also doubled. Doubling one or the other produces the same length curves but with different curvature.
Switches
Prototype railroads measure switches by numbers. This is directly related to the
MSTS measurment of degrees as the 'number' is simply a different way of
measuring the angle of the switch. Look at this picture: 
The number N is the distance (in yards) until the two tracks are one yard
apart. Naturally the distance can be measured in meters or feet until the
distance is one meter or one foot. You can calculate the number from the degree
of the switch like this: N = 0.5 / TAN(D).
This table lists MSTS switches with their radius and approximate numbers.
| MSTS switch | Degree D | Number N | Radius R [m] |
|---|---|---|---|
| A1t30dTram | 30 | -1 | 18.604 |
| A1t45dYard | 45 | -1 | 60 |
| A1tPnt6d | 11.282 | 2.5 | 101.2882 |
| A1tPnt10d | 10 | 2.8 | 164.0639 |
| A1tPnt3d | 6.322 | 4.5 | 271.64074 |
| A1tPnt5d | 5 | 5.7 | 655.007 |
| A1t870r4d | 4.338 | 6.6 | 870 |
| A1tPnt2_5d | 2.5 | 11.5 | 2618.7818 |
1) The number for tram and yard points is not meaningfull.
2) Degree for the modular points are for a complete switch or the newer one
piece switches.
Please note that the number for MSTS switches is somewhat missleading. This is because the diverging track continues to curve away from the main track after the frog. The angle should be measured at the frog and therefore would be less than the MSTS degree. Usually prototype switches does not curve after the frog though there are excpetions. Because of this the number value for MSTS switches is less than a comparable prototype switch. To tell the truth they are more comparable to a prototype switch of x2 the calculated number. The XTracks modular points (3d & 6d) are closer to prototype than are the original MS/Kuju points.