Well, for railroad ties (or sleepers, as they are called in the UK), you may be happier getting some 3/16 x 1/4 basswood than with the balsa--I find the balsa is simply too soft too often. You can also get weather-It & Stain-It which will give basswood the look of either aged, or creosote-soaked lumber--a real plus.
That being said, I do have a use for balsa--it's good for making rotted ties, because it's so soft. You can take a bit of brass or plastic and "mash" a flat where the rails cross; a needle in a pinvise can be used on the end grain to give that rotted out look. The needle will also scribe the sides to give that old distressed look. To finish, dip in black poster paint, then highlight with a deep brown.
Next time you are in the LHS, grab some stumps from the WS line, and use them to "frame" where the ties/timbers get piled. Timber and lumber piles can be good cover, which attracts infantrymen being cautious. Ammo box with a sinkhole that might be otherwise un-usable super-glued at a "toppling" angle could give a sense of the "reason" to be cautious, too.