In theory any switch can be mated with any type of flex track, but in practice there are limitations. For many brand combinations the work to connect the two types of track is onerous and kludgely. You might, for example, connect two incompatible types of track in a few places for, say, a transition from staging to the main layout or from a main line to a branch line, but not as something you'd do for every switch. In addition, there is an appearance question, as certain lines of switches look out of place when connected to certain lines of flex track.
The purpose of this post is to cover just those brands of N scale flex track and switches widely available in the U.S. Regarding the below references to track "codes", see my earlier post on that topic.
Atlas Code 80. The granddaddy of N scale track, now distinguished from other track lines by its cheap price (good) and unrealistic appearance (not so good). A wide variety of switches are available, but nothing larger than a #6. There is also a wide variety of crossings and sectional track. Some people still use Atlas code 80 because they have experience with it and know it works. Others choose it for staging and hidden track, where appearance is not important.
In addition to appearance the common complaint about Atlas code 80 is lack of switch reliability. This can be addressed with certain switch prep procedures, as I've covered in an earlier post, and in addition Atlas has improved the switches in the past couple years.
Atlas code 80 can be easily mated with all Peco track.
Atlas Code 55. Probably the most popular choice for N scalers using flex track in North America today. This is because it is price competitive with the other options, except Atlas code 80, yet the appearance is much better. There are also a wide variety of switches, crossings, and sectional track available in this line, including a #5, a #7, and a really cool looking #10 switch. Atlas also just announced two curved switches. There are still no slip switches, but given the number of recent extensions of the code 55 line that may change.
Criticisms of Atlas code 55 are: 1) the plastic molded "spikes" which hold the rails in place are oversized, 2) because of (1) cars which run the Micro-Trains "pizza cutter" wheels (that is, those with a high flange profile) can't run on Atlas code 55, and 3) although the switches have a nice wire for the frog, it tends to corrode and lose contact, so you still have to add your own frog wire. (See earlier post about frogs and switches.)
None of these criticisms are severe. For (1), while the spikes are oversized if you look closely, from a distance it's not noticable. For (2), most people who use Atlas code 55 are likely to use lower profile wheels which avoids the problem. And for (3), this can be taken care of easily during installation.
Atlas code 55 mates reasonably well with Micro-Engineering code 55, so some people like to use M-E flex track with Atlas switches.
Micro-Engineering (M-E). Micro-Engineering prides themselves on making high quality products, both in terms of appearance and reliability, and that is a good description of their code 70, 55, and 40 lines of N-scale track. They offer these with weathered or non-weathered rail, and with concrete ties for the larger two sizes. They also are the only manufacturer to offer a bridge tie version of their flex track, which is important because bridge ties are very different than normal track ties (roughly twice the tie density, and a different shape). All this variety allows a modeler to include different rail sizes for different areas of the layout, as is typical in the prototype.
M-E was historically the most expensive track but that situation is no more. The UK firm Peco raised prices extensively in the 2000s, in part due to the declining dollar, and now is more than 50% higher than M-E. Based on my recent price samples M-E now seems roughly in the same price range as Atlas code 55 flex track (Atlas has also had a few price increases on track in recent years).
The only complaint I've ever heard about the track itself is minor -- a few people say it is harder to bend than the other brands. The main complaint about the line of track, other than the price, is that the switch offerings are so spartan: just a #6 switch for code 55 and 70, and if you want a code 40 switch you have to make it yourself.
Because of the superior appearance of track but lack of switch choice many modelers use M-E code 55 with Atlas code 55 switches, and the two rail sizes match up reasonably well. However, if you want the epitome of best appearance for your track, the choice is probably M-E plus hand-laid switches, and for that M-E also offers supplies for those who build their own switehcs.
Peco code 80. For a while the Peco code 80 line was considered the best choice in track, before the other sub-80 lines were introduced. It is considered somewhat better in appearance than Atlas code 80, and their code 80 switches are considered much more reliable than Atlas code 80. Because the Atlas and Peco rails work well together it used to be common for modelers to use the cheaper Atlas code 80 flex with the better Peco code 80 switches, and some Ntrak clubs still use this as their standard.
Peco also offers a wide variety of switches, including the only slip and curve switches in code 80, as well as crossings, sectional track, and even a derail. (Note: there are actually two compatible lines of Peco code 80 track -- the cheaper SEtrack and the more expensive Streamline. I'm only covering Streamline in this review, as SEtrack is very hard to get in the U.S.) Their switches include a "spring" for snapping to one side or the other, which can be removed to install a slow motion switch machine. In code 80 they also offer "electrofrog" and "insulfrog" for all switches.
Complaints about Peco code 80 switches are primarily from the American market because the UK N scale standards vary every so slightly from US standards, so that depending on the equipment you run you may have trouble with derailments or, if you use DCC, with occasional shorts. These seem pretty rare, but you do find them mentioned on comment boards, and there are standard suggestions for fixing this problem if you have it.
The other issue with Peco, as noted earlier, is that the prices have skyrocketed in recent years and as a result they have nearly priced themselve out of the U.S. market.
In HO, Peco has released American-style switches that are considered among the best you can buy, yet they have managed to keep the prices within reason. There have been rumors of a similar line for N scale, but nothing has yet been announced.
Peco code 55. This line of track is highly unique, and so it has a longer write-up than any of the other lines. When Peco introduced this line in circa 1990 it was widely considered a huge advancement and the preferred choice for N scalers. Today, most modelers now see it as having been surpassed by M-E and Atlas, but there are still some unique advantages of Peco code 55, so it is not yet completely obsolete.
When Peco code 55 was introduced there were a lot of commonly-held concerns about the viability of a small N scale rail. Peco addressed this by using code 80 rail but burying the bottom .25" into the ties. The track itself is very sturdy -- more than any other of the flex track offerings -- and the plastic tie "spikes" or "clips" are prototypically small, as they are used only for appearance, not to hold the track in place. There is a fake rail bottom at the top of the ties, then another one at the actual bottom of the rail. Peco code 55 track comes in wood or concrete, but just wood for the switches (AFAIK no one has yet come out with an N scale concrete tie switch yet).
Peco 55 has almost the same large variety of switches and crossings as code 80, but all are "electrofrog". There is also a double-crossover with four built-in switches. Like code 80 the code 55 switches have springs which can be removed for slow-motion switch machines.
Proponents like Peco code 55 for solidity, reliability, and wide variety of switch choices (this has become less of an advantage over time as Atlas has expanded their code 55 line). Detractors, and there are many, point to the high cost and appearance issues.
Appearance issues start with the tie spacing, which matches a European prototype, not the U.S. The wood ties have a fake wood grain that is much too prominent. The switch mechanisms look nothing like prototype switches. The switches themselves are classified "small", "medium" and "large" instead of the more traditional #4, #6 and #8. This is because they do not follow the common U.S. practice of having the diverging rails go straight through the frogs, but instead are curved for the entire length of the switch. On the plus side, this difference means that the radius used for Peco code 55 switches is much wider than that use for competitive brands, and that the trains motion in going through the switch is more fluid. The "large" switch has a radius of 36", which is very broad (see earlier discussion of curves). But it is yet another deviation from the common American prototype.
Finally, there are questions about code 55 switch reliability (there are concerns that the flangeway is too wide, leading to derailments), although there are many users who claim they work perfectly all the time.
Given the cost issues Peco code 55 is usually not considered for new layouts any more. But thre are still a few willing to work through the appearance issues who find the variety of switches a compelling reason to choose this option.