I originally thought it was an oligopoly, but then i read that about 50 companies make up over 99% of the market. I know that would make it monopolistic, but my question is, do 3-5 of those 50 control over 80% of the market, thus making it an oligopoly?
As any good economist will answer, "it depends." ; )
Yes, a small group of firms that can control profitability in a market are an Oligopoly. The first issue is to define what the market is (not as easy as you might think). Next you have to think in terms of profit, not necessarily market share (although i'll address that too via a discussion of the Herfindahl index).
Let's skip the market discussion until after looking at control over profitability. From a regulatory perspective, the issue with an oligopoly is whether it controls long-term pricing. Unless the oligopoly is profitable, it cannot sustain long-term pricing against consumers and is not likely to raise regulatory pressure to change. Having lots of market share, OTOH does not necessarily translate to profit (Amtrak on the US east coast is a near monopoly for passenger rail traffic over long routes but runs at a loss with no better rail alternatives).
To understand command over market profits, it's useful to consider the "Porter's Five Forces" model, where industry profits come from the following: (i) bargaining power of suppliers, (ii) bargaining power of consumers, (iii) degree of competition (within the market), (iv) barriers to entry for new competitors, and (v) substitute product/service providers. As a sixth force, consider regulation (which allowed ethanol producers to be profitable only via government subsidies). If an oligopoly is controlling any of these forces to the point where industry profitability is being manipulated to the favor of the oligopoly, regulators will likely step in to correct the situation.
Coming back to the issue of the market, think about railways in terms of the substitute product item from the Porter model. For passenger rails, you compete with buses and planes. For freight rails, you compete with air, ships, and trucking. In this context, an oligopoly is more benign.
Finally, the question of market share (% of total sales in the market) can be measured by a cool statistic called the the Herfindahl index (HI). The HI is used by the Dept. of Justice (in the US) to determine if the acquisition of one industry player by another is "increasing market concentration". In other words, is the merger likely to increase long-term market pricing in a way that is adverse to consumers? The issue defining the market is still important. Even though the Sirius' acquisition of XM satellite radio merged the only two satellite radio service providers, consumers have bargaining power in that there are many alternate "audio" substitutes in the event that satellite radio becomes too expensive i.e., terrestrial radio, CDs, i-pods, etc. (not to mention that satellite radio is not a necessary consumer good by any stretch). The DoJ approved this merger.
To calculate the HI, you take the market share of the top-eight companies in the industry (by market share; any more than eight will not change the calculation much). Square the individual market shares (multiply them by themselves) and then add the results. This is your measure of "market concentration". The DoJ likes to see the HI below 0.10 and will raise eyebrows if a merger increases HI over 0.01.
As an example, consider a market with three players with 33% of the market each. The HI is (33%*33%)+(33%*33%)+(33%*33%) or 0.3267. If two firms merged, the math would be (66%*66%)+(33%*33%) or 0.5445, a big difference.
This is not a straight answer to your question, but the real answer depends on why you ask and is not straight forward. Hope this helps!