I'm an avid cyclist, primarily a mountain biker, and am always looking for new & better ways to train. I live in Colorado and my favorite trails are packed with snow a good part of the year and are often muddy at other times of the year. The area's paved roads are (mostly) open year-around but they don't provide the workout that a ride up a 15-20 degree trail provides.
It occurred to me that a bicycle trailer carrying weight would allow me to ride on paved (preferably hilly) roads while getting the same exercise I would normally get on my favorite (but frequently inaccessible) trails. So I Googled for such a beast and was very happy to discover CycleTote's Bare Trailer which, with a bit more reinforcement, turned out to be exactly what I was looking for.
I purchased the trailer and then added a plastic storage box (as suggested for use with the Bare Trailer) and two plastic 7 gallon Aqua-Tainer water containers. The trailer was then integrated into my training regimen. As I used it, I added a few of my own custom features. My conditioning improved significantly as I used the trailer over the next few months -- not to mention that, combined with better dietary habits, I lost 40 pounds!
I talked to the folks at CycleTote about offering a trailer product geared specifically for training. They agreed with my suggestion and the name TrainingTote was adopted . They asked if I would be willing to share my modifications and experiences, so here they are:

Packing of Water Containers in the Storage Box -- To keep the water containers from jostling around too much, I use some spare bubble wrap as padding between the containers. In addition, I stuff my extra clothing around the water containers which adds further padding.
My Experiences: I've found I get more consistent exercise going up a nearby paved road with the TrainingTote compared to going up my normal mountain bike trail sans trailer. This is because my mountain bike trail, like most trails, has a variety of different grades (including flat stretches) while the paved road has a very consistent grade. So, with the TrainingTote, it's a constant burn all the way to the top of the hill whereas my mountain bike trail grants me periodic "reprieves" along the way. So, for continuous exercise, it's hard to beat the TrainingTote and I've actually found myself preferring to use it on my favorite paved road compared to mountain biking on some of my usual trails.
Note that you don't need a steep road to get a lot of exercise. The TrainingTote turns even a moderately-sloped road into a grueling training course.
Two ways to use the TrainingTote that I've considered are: (1) Keep the trailer weight constant and attempt to achieve better times on a specific hill climb or (2) establish a baseline time on a hill with, say, 40 pounds of water, and then attempt to maintain that time as I increase the water weight. I've been doing the first approach (improve my times) which I believe contributes more to my cardiovascular endurance. I believe the second approach (increase the weight) would contribute more to leg strength.
One possible training scenario would be to focus on leg strength during the off season (hence, the second approach) and then switch over to the first approach to optimize endurance as the biking season begins.
There's actually a 3rd method of using the TrainingTote that I've experimented with. Whereas weightlifters have coined the phrase "lift until fail" (meaning they do as many repetitions as they can until they can't continue), I've used the TrainingTote to "bike until fail". I pick a really steep (but short) hill and load the TrainingTote with sufficient weight such that I can make perhaps 5-6 trips up the hill. At some point, I'm unable to continue riding simply because I no longer have the strength/endurance to continue to pedal the bike & maintain balance -- hence, "bike until fail". I've found this method of conditioning leads to being able to quite rapidly either: (1) increase the number of "reps" (trips up the hill) or (2) increase the trailer weight while holding the number of trips at around 5 or 6.
-- Dave