Geof says:

The switch numbers you mention refer to how sharp the angle of the diverging track is from the straight or main line. So a #4 switch has a sharper angle than a #6 or #8.
To determine what switch size to use you first should think about the following things:
1.) What type of traffic will be passing over the switch? Will it used for an industrial siding, in a classification yard or on a mainline?
2.) What type of rolling stock and locomotives will be you be running? Generally, shorter cars (like 40′ & 50′ freight cars) and smaller locomotives (like steam and diesel switchers) work fine with #4 and higher switches. You run into trouble when you are running very long equipment (like passenger cars or big steam locomotives) because their length makes it hard for them to negotiate the sharper turns of a #4 switch which may result in frequent derailments. Most equipment will operate on #6 switches and everything will run on #8.
3.) Another consideration is room. The smaller numbered switches take up less room than a larger one because they make the turn in a shorter area and are actually shorter in length as a rule. Wye switches are just that, a Y. The diverging tracks ‘split’ apart equally from each other. As with other switches, they can come in various sizes like ‘normal switches #4, #6 and so on.

A single slip and double slip switch is almost like having three or four switches ‘stacked on top of each other to form a ‘super’ switch. Think of a figure eight track plan ( 8 ). The double slip switch would be where the upper and lower loops merge. By using a double slip at the merge point, your train could just run around the upper loop OR the lower loop OR the whole figure eight depending on which way the double slip switch was set up. And you can switch back and forth over each of the pathways I described without stopping your train. Generally though, a double slip switch isn’t used in a figure eight setup. I just used the figure eight to easily explain how the slip switch works. Its application is usually in a railroad yard where there isn’t that much room to have four switches spread out where the double slip does it all in the area taken up by one switch.

A single and double crossover is a switch set up that allows a way for two or more tracks that are parallel to one another to connect so a train can crossover from one line to the other. The single crossover would look like the letter N where the angle part of the N would be the crossover connecting the two tracks. So, if you had two inner and outer loops of track with ONE crossover, you could cross from one to the other but once you made the change, the train would be ‘stuck’ on the loop it crossed to.
A double crossover would look like the letter X but BOTH tracks could be accessed by the other by using the crossover. So, as in the above example, if you had a double crossover, you could run trains back and forth from each loop.

Hope this helped.

Geof

Robert Anderson says:

Jerry, Geof might not see your follow-up question here. You would be best advised to submit your question at http://www.model-railroad-resources.com/submit-questions.html and have it published on page 1.