Levels adjustment is likely to be one of the first refinements you will want to carry out.

If the histogram doesn't stretch all the way across the horizontal axis it means that the full range of tones is not being used in the image. The blacks are not as black as they might be and the whites are not pure white. The basic levels adjustment is to ensure that the white slider (on the right) and the black slider(on the left) are moved in to enclose the histogram. On the example shown on the left the white slider should be moved in quite a way and the black slider a little. This can be done manually or by clicking the auto button. The central grey slider can then be moved to adjust the mid tones.

At the bottom right hand side of the levels dialogue are three eye droppers, From right to left to set white, grey and black points. When using these it is useful to have the info palette visible. If the white eye dropper is moved over the image to the whitest point in the image the R, G and B values should be about the same. If they are not then the whites have a color cast. Clicking the eye dropper sets the white point and corrects this colour cast. Using the black dropper in a similar way sets the black point.
The grey dropper is used to correct colour casts, choose a neutral spot (without colour) and click on it. The colour balance of the whole image is adjusted based on the fact that you have told it what neutral should be.
If you are not sure where the lightest and darkest areas of the image are these these can be determined by pressing the Alt key and dragging the black and white sliders inwards. This method can also be used to position the sliders at the extremes of the histogram.

Advanced Levels Adjustment

Carrying out a simple levels adjustment as described above will improve the image but it is unlikely that optimum results will be obtained for the whole image. The next section shows how to use a number of levels adjustment layers to optimise particular areas of the image

This is Antony Guppy's original image as scanned. It is a very pleasing image with an attractive sky but I felt that improvements could be made.

I first removed the wires using the patch tool in Photoshop 7®

To finalise the image a third levels adjustment layer was used in the same way to slightly darken the locos, with a layer mask used to mask all the image exept the locos. This was done by painting with black everything but the locos. Alternatively the layer mask could be filled with black and the area of the locos then painted with white to remove the mask in that area.

When using multiple adjustment layers it isn't necesarry that they all be the same type. It may be effective for example to use a combination of levels, curves and hue and saturation adjustment layers to produce the optimum result.

To increase or decrease the amount of detail you can see in shadows midtones or highlights choose Image>Adjustments>Curves. To change the tonal range click on the curve to create a point; then drag the point to change its position. The rest of the curve will change shape to make a smooth transition. A curve such as that shown left will darken the shadows and lighten the highlights.
If there is a particular tone that needs to be altered then open the curves dialogue, take the cursor into the image and click on the tone to be changed. You will notice as you click an indication on the curve showing where the tone is located. It can then be lightened or darkened by dragging that area of the curve.
The three eyedroppers at the bottom right of the curves dialogue box work in a similar way to that described under 'levels'.