Are the fast trains from Rome to Milan reliable when planning itinary?

Taking the train from Rome to Milan is easier than flying. It is direct city center to city center, no need to arrive 2 hours early for security, etc. Buy tickets in advance for a discount but know that you must then take that exact train. Be sure you book either a high-speed Frecciarossa train on Trenitalia or one of the Italo trains for the best experience. Not sure from your post of your final destination; Are you are going directly to Lago di Garda the same day?

Spend your time in Rome, get to the station 20 minutes before departure time and enjoy the ride,

Our destination is Lake Garda and thanks for the input-much appreciated!

Rail workers in Italy do occasionally go on strike. Usually for a 24-hour period, during which many trains are cancelled. If it was me, I would plan on taking the train to Lake Garda two days before the start of your tour, giving you another day to get there if there is a rail strike.

The most unreliable part of the trip is probably the arrival time in Rome due to flight delays. Instead of building in a buffer of several hours waiting in a train station what we like to do when taking long-distance trains is arrive the day before. Enjoy a bit of Rome and then take the train from Roma Termini in the morning on a cheap advance-purchase ticket.

Strikes can be a concern but are less likely than plane delays. But even on strike days there are guaranteed trains. Use translate to look at the page https://www.trenitalia.com/it/informazioni/treni-garantiti-incasodisciopero.html at "Long-distance trains guaranteed in the event of a strike". That's another advantage of arriving a day in advance. Your flight might not arrive in time to catch the train but if you're in Rome the nite before then you'll be able to.

Flying into Rome is a fine option. You will need to get from the airport into the city. If you are going to take a train then taking the Leonardo Express train (33m) probably makes the most sense since it drops you off at the Roma Termini (the main train station) where you can catch high speed rail onto your destination. The options from Milan are very much the same as both airports are outside the city. Milan is a little closer but the difference is probably a couple of hours on the train so it's up to you.

Now the big question is where are you going? I would suggest Verona as the perfect place you take the fast train and get your bearings. It is a very pretty, easy to navigate city and is the jumping off point for train and bus (for the east side) to Lake Garda.
Verona is 3:30 from Rome and 1:13 from Milan with many options during the day.

Whether you want to try and tackle the train after your flight is up to you. Staying overnight in Rome/Milan is not a terrible idea if your flight options are long. (I fly from San Diego and it is a trek.)

If you are going on by train it is easiest to wait and buy you tickets after you land and clear the airport. There is a long, recent thread on buying tickets ahead of time. If you decide to stay overnight you can prebuy your tickets for the next day by train.
https://community.ricksteves.com/travel-forum/italy/buying-train-tickets-on-day-of-arrival

The bottom line is you can't predict whether your flight will be on time, how long luggage and immigration make take so buying ahead of time means you might miss your train unless you buy one much later in the day and then end up waiting. Between the two companies there are so many trains just wait until you're ready and buy the next one.

Yes you can use your credit card but by far the easiest way is to use the app. You can preload it with your information on the website account and then buying tickets takes a minute or two.

There are two train companies in Italy - the state run Trenitalia and the private run .Italo that only runs fast trains between big cities.
https://www.trenitalia.com/en.html (Use Italian names for cities - Firenze for Florence and Roma for Rome - for best results)
https://www.italotreno.com/en
Fast trains have reserved seating, multiple classes and you can buy tickets ahead of time at a (sometimes significant) discount trading flexibility for price.

Both companies have apps that let you buy and store tickets as well check trains in motion. I recommend getting at least the Trenitalia one if not both.

A great resource for train travel in general is here: https://www.seat61.com/train-travel-in-italy.htm

Hope that helps, have a great trip,
=Tod

Tod has provided excellent advice.

It's not clear to me what you plan to do on arrival- whether that is Rome or Milan.
Will you stay in one of those cities to get over jet lag- then head to Lake Garda?
Where exactly on Lake Garda does your hiking tour start?

Trains are certainly reliable and very easy.

If the flight to Rome appeals to you but you don't want to stay in Rome - go to Verona on arrival- direct fast train from Roma Termini to Verona Porta Nuova
From there you can head to your hiking tour. You may need to figure out this transportation- bus or car if location is not on train line
The fast (and regional) trains only service Desenzano del Garda- which could also be your place to get over jet lag if you are not interested in Verona (we loved Verona- lovely little city to spend a few days in)

If you fly in to Milan but don't want to visit Milan then go right to Desenzano or Verona. Or anywhere in the area that appeals to you- Bologna, Bergamo, are also options- both with train service to Desenzano

IOW- You do not need to go to Milan at all if you fly in to Rome.

Whatever you decide- don't purchase onward travel train tickets for your arrival day. Just wait til you are there - delays do happen. Trains won't sell out so you will just be paying the Base Price.