The western trailhead of the Issaquah-Preston Trail is 100 yards or so up the East Lake Sammamish Trail from that trail's southern trailhead. So begin there, at NW Gilman & the east leg of 4th Ave NW in Issaquah. (The east leg of 4th Ave NW - a one-way, one-lane street starting just east of the caboose on Gilman - sports a lane of free parking along its west side that's mostly used by Trail cyclists and walkers.) The East Lake Sammamish Trail itself is adjacent, just to the east of 4th Ave. Cycle under the I-90 double underpass to see the Issaquah-Preston Trail's entrance on the right.
The only serious road crossing is early on, Front Street. With its I-90 entrance and exit ramps, you’ll want to use the crossing light! From there, you’re headed uphill to Preston, mostly along I-90. The trickiest choice comes quickly, the fork. Choose the LEFT fork, alongside the vegetative sculpture, under the four- or six-lane highway-speeds Highlands Drive to the immediate T, where you'll choose left once more; you’ll be off pavement onto gravel just a few yards further.
If, instead, you find yourself past the fork, still on pavement, on a relentlessly-brutally-uphill trail, wondering what train system would have tackled a grade like this one, you’re asking the right question - no train would have - you took the right fork, as I did - and you need to coast back down to the fork again, to take the correct fork, past the sculpture, under the for-all-intents-and-purposes highway (Highlands Drive), then left again onto gravel, to stick alongside the I-90 right-of-way.
As others have reported, there’s highway noise along much of the Issaquah-Preston Trail - but I’d characterize it as considerably less noisy than the Cedar River Trail, which I will not cycle again. This trail angles through lots of forest before it emerges onto SE High Point Way which, on a late Saturday afternoon, had zero traffic for the almost-mile it shared a roadbed, with bikes getting a full car-width lane set off from vehicles, had there been any. It then seamlessly becomes (well, you do have to cross to the north side of High Point Way at the trail crossing sign) the paved Preston-Snoqualmie Trail, wending its way past Preston’s trucking and light industry firms, then its athletic fields and park, before easing back into forest.
The fields and park are the high point of the trail; from here begins a gentle descent. I turned around two miles past the park, when the paved trail took a sharp turn to the right into what appeared to be a much steeper descent - I’d had my quotient of climbing in my inadvertent detour to Issaquah’s Highlands. But before reversing course, I continued on gravel straight for 100 yards to what must have been the beginning of the railroad’s Raging River trestle. There’s just a bench here, now, with the sound of the Raging River far below.
My total roundtrip was 19 miles - not sure how much of that was my “adventure” - as steep as it was, I’m sure it seemed much longer than it was - probably roundtrip, less than 2 miles off my intended route.
