Having ridden this trail in sections numerous times, and having ridden the entire length in a day last fall, It's my opinion that this is the finest rail trail for cycling purposes in NH.

In Merrimack county (Boscawen to Danbury) the dedication of the local friends group is evident. The trail is well maintained and the surface is excellent. From Grafton to Lebanon (Grafton county) the surface is trickier, with regular sandy spots and some exposed railroad ties. Despite this, the trail is still very ride-able in a gravel bike, and my wife had no issues in Merrimack county on a cruiser/townie bike.

Really enjoyed the trail. Great views.. sun and shade . We will be back !

Go biking here quite often a lot of shade good when it's hot

I have found a nice way to link to the Windham Depot to Derry Trail. At the southern end, cross over Rockingham Road onto Seasons Lane and turn left onto Coteville Rd. At the end of Coteville there is a short path through the woods that comes out on Franklin St Ext. Follow Franklin and turn right onto Mitchell Ave. At the end of Mitchell is a path that goes to the Windham Depot.

I will preface this by saying I'm basing my opinion on being a runner. It is probably a great trail for dirt bike riders, ATV riders, snowmobilers, etc. For runners the surface is soft in places, pitted in places, has lots of rocks in places, and overall is just not a great surface for runners.

We bicycled a sort of loop, from Mosely Woods parking lot, along Merrimac River, crossing to Salisbury and then returning via I-95 bridge bike path. The latter was our true goal, as this was a mighty project! From the start and throughout, we had some head-scratching moments trying to find our way on the network of rail-trails and roads. I'm not sure if it's the fault/responsibility of the municipalities, the DOTs, or the trail organizations, but there are few, if any, signs indicating bikeways. This struck us as odd because in many other areas throughout Essex County in Massachusetts, and in parts of NH and ME, we have seen copious signs for the East Coast Greenway in remote countryside locations.
Before I say more on that, we enjoyed the gravel-paved Marsh Trail and Ghost Trail. The latter was shaded and welcome on a very hot summer day. The mile or so of asphalt-paved trail that parallels I-95 and crosses the Merrimac is great for getting from A to B, but it's open and not especially pleasant except for the river views, which can be enjoyed from two bump-outs on the bridge and include historic interpretation signs. (A note: if you are nervous crossing major bridge structures on bicycle, this is a good crossing. I have minor such fears, but felt completely comfortable.)
Back to signs. There are signs. But I don't think they are always where they should be, nor say what they should say. More than once, we came to intersections of various kinds and it was unclear where each option would take us. In some places there are posts with trail names and mileage., but few signs to provide context. Within long stretches of the bike paths there were posts indicating mileage (for what reason, I have no idea). Coming from the Ghost Trail there was no explicit sign to point the way to the I-95 crossing. And in Newburyport, we did not see any signs pointing the way to the trail, crossing via the Route 1 bridge. Coming off the I-95 crossing into Newburyport, we had to intuit the way back to Mosely Woods on a neighborhood road.
My suggestion is that if you are solo or with a partner, have fun and explore, and if you have to do a bit of back-tracking, no big deal. If you have kids, it might be better to thoroughly plot your route, as you don't want to be fumbling on roads with cars with kids or a larger group. That said, it would be a nice adventure to cross by two different bridges and experience a variety of trail types and scenery that runs the gamut from marsh, woods, industrial (including a massive solar array), neighborhood, etc. It is almost all flat, and eminently do-able for people of various fitness levels and bike types.
A note: the Route 1 bridge does not have a bike lane, so you must walk your bike across on a sidewalk, and will likely encounter others either on bike or foot in either direction.

Bikers beware. As others have noted, this is a narrow trail with rails still in place. It is a beautiful path and perfect for walkers and runners. If you choose to ride, Wear a helmet and heed the signs to walk across the on\off ramps where the path veers from the rails, which is often. As others have noted, bike tires can become snagged and cause you to be thrown from your bike. This happened to my partner requiring fire dept/emergency rescue from the trail. Shame on us for not wearing helmets. Concussion and fractured arm later, all is well. Emergency responders and local hospital were all amazing - especially given the current concerns in healthcare. Important to note, no one was surprised at what had happened, it’s not uncommon. Again, beautiful trail, but I think this is a fair warning if you do choose to ride this trail.

My son and I love riding this on our dirt bikes. Wide and easy to ride, perfect place for a new rider to get comfortable. Plenty of other people on bicycles, hikers, ATVs and dirt bikes on the trail, everyone was friendly and courteous to give each other room. It’s nice having a place like this to spend time with my son.

If you are parking on the Farmington side, get ready to get creative with your parking although, if you intend on riding it, it's probably the safer bet since you do need to cross a MAJOR roadway about 1 mi from the trail head.

As the only person riding the route with a full suspension fat bike, I found it tolerable but the rest of my group were not as fortunate. The sand was soft, or rocky, or a combination of the two. There were parts of our bodies forever damaged (kidding... kind of) as we literally pounded our way down this viewless trail. Of the six miles we rode, probably 1.5 was "doable" on a regular bike otherwise it was just extremely difficult to navigate.

There is no river..... but there is a bit of a cess pool looking pond. We went in the middle of a drought which probably worked to our advantage as any water on this trail would make it nearly impassible.

After six miles, we decided enough was enough so we chose to take the road back. Mind you... riding along Route 11 was our only choice (think massive commercial district with significant traffic)... and yet.. it was still better than trying our hand at that return on that trail. Thank goodness we were all relatively adult... at least numerically... taking a kid on this would be a HARD no.

Unfortunately, we see a real opportunity to for rail trails but if the funding isn't there to maintain it then it's not going to get much use. That was the case with this strip.... and, hopefully with some resources, it can meet its potential of being a fun day of recreating.