1. As patinba and kpdinicola have already indicated, "S." is part of "Curvas en S.", which is fairly self-explanatory and means S-curves or S-bends. Both these terms are valid, but the one that seems to be most commonly used in relation to railways is "reverse curve". It's also known in Spanish as a "contracurva".

"In civil engineering, a reverse curve (or "S" curve) is a section of the horizontal alignment of a highway or railroad route in which a curve to the left or right is followed immediately by a curve in the opposite direction."
http://en.wikipedia.org/wiki/Reverse_curve

Mario León, whose Diccionario del tren is a standby for railway questions, uses "reverse curve":
http://books.google.es/books?hl=es&id=et6IRQ60RO0C&q=curva e...

But as I say, either "S-curve" or "S-bend" would be fine. By the way, we had a question on "S-bend" in relation to roads, which I answered:
http://www.proz.com/kudoz/english_to_spanish/automotive_cars...

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2. An "escape" is not a siding. At least, not usually, although kpdinicola's Mexican source seems to be describing a siding. Normally it's what's known as a crossover. Mario León gives "exhaust", but I think that's a mixup; he gives "crossover" for "escape de enlace" and "double crossover" for "escape doble":
http://books.google.es/books?hl=es&id=et6IRQ60RO0C&q=curva e...

" Agujas dobles o escapes
Un escape, o aguja doble, es la unión entre dos vías paralelas."
See accompanying photo. This is a great page, by the way. "Agujas" are points:
http://comofuncionanlostrenes.blogspot.com.es/2014/01/aparat...

Mario León's definition of "escape" confirms this:
"aparato de vía que pone en comunicación las circulaciones de dos vías, generalmente paralelas, mediante dos desvíos con la misma tangente y con sus ramas desviadas en prolongación una de otra".

For desvío ("switch"), see:
http://es.wikipedia.org/wiki/Desvío_(ferrocarril)

In English:

"A crossover is a pair of switches that connects two parallel rail tracks, allowing a train on one track to cross over to the other. "
http://en.wikipedia.org/wiki/Railroad_switch#Crossover

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3. "cruce" is less clear. It can mean a level crossing, but I don't think it does here. I think it's just a crossing: in other words, a point where one track crosses another. A crossing has a crossing frog, which is a specially shaped piece of rail, but here I think they're talking about the actual crossing of tracks rather than the frog itself:

"The frog, also known as the common crossing (or V-Rail in Australian terminology), refers to the crossing point of two rails."
http://en.wikipedia.org/wiki/Railroad_switch#Frog_.28common_...

I'd put "crossing".

What all three of these things have in common is that they involve curves that diverge from a straight line, which is why the "radio mínimo admisible" is relevant.

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Note added at 3 hrs (2014-11-17 22:33:00 GMT)
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Forgot to say: the normal word for siding in Spanish is apartadero. Apparently in Mexico it's called a ladero o escape. If your text is from Mexico this is the translation, but not otherwise.

In Spain an escape is definitely a crossover, not a siding. Here's further confirmation from an ADIF document (see p. 12):
http://www.adif.es/es_ES/conoceradif/oferta_de_empleo_public...