Railway vs Fly.io in 2026: Secrets, Isolation, and What Each Actually Costs

Railway hands your database password to npm install. Fly.io doesn't. That single difference in when each platform injects a secret is the most security-relevant thing separating these two platforms, and almost every comparison you'll read skips it to argue about pricing tiers instead.

Railway's own docs are explicit: variables "are made available to your application as environment variables" during "the build process for each service deployment" and "the running service deployment." Fly's docs are equally explicit in the other direction: "If you need secrets to be available when building your Docker image, see Build secrets." Same word, opposite blast radius.

Platform Overview

What Is Railway?

Railway is a deployment platform focused on developer experience. It runs applications in Docker containers with a visual interface for managing projects, environments, and services. Railway emphasizes quick deployment from Git repositories with minimal configuration.

What Is Fly.io?

Fly.io runs applications on Firecracker microVMs, the same technology AWS uses for Lambda and Fargate. Applications deploy globally to edge locations for low latency. Fly provides more infrastructure control including dedicated IPs, private networking, and custom machine configurations.

Security Feature Comparison

Secret Handling: The Part That Actually Differs

Both platforms encrypt secrets at rest. Both inject them as environment variables. The interesting question isn't whether they're encrypted, it's when they're live and who can read them back.

Railway: variables reach the build

Railway variables are available in two places, and its docs name both: the build process for each service deployment, and the running deployment. Sealed variables are no exception. Railway describes a sealed variable's value as "provided to builds and deployments but never visible in the UI nor can it be retrieved via the API."

Sealing protects the value from a human with dashboard access. It does not move the value out of the build.

If you're on Railway, the fix isn't to leave the platform. It's to know which variables are actually needed at build time (usually a public API base URL, sometimes a private package registry token) and stop putting the rest anywhere the build can see them. Our Railway secrets guide walks through separating those two sets.

Fly.io: secrets are runtime only

Fly's split is the one you'd design if you started from the threat model. fly secrets set puts a value in an encrypted vault. At deploy time, the Machine gets a temporary auth token, decrypts the secret, and injects it as an environment variable in the running VM. The build never sees it. Fly's docs send you elsewhere if you need one during docker build, to a separate build secrets feature you have to opt into per value.

Fly is also stricter about reading values back. Its docs put it plainly: "For security reasons, we do not allow read access to the plain-text values of secrets." Railway's plain variables are visible in the dashboard, which is convenient right up until you're auditing who on the team has seen your Stripe key.

Isolation Security

Railway Container Isolation

Railway uses Docker container isolation with standard Linux namespaces and cgroups. Containers are effective for most workloads but share the host kernel with other tenants. Railway's infrastructure adds additional security layers, but container escapes remain a theoretical concern.

Fly.io MicroVM Isolation

Fly.io uses Firecracker microVMs, which provide VM-level isolation with a minimal attack surface. Each application runs in its own VM with a dedicated kernel. This stronger isolation is particularly valuable for multi-tenant platforms or applications handling sensitive data.

Network Security

Railway Private Networking

Railway services in the same project communicate over private networks without internet exposure. Internal DNS resolves service names automatically. This simplifies secure inter-service communication but is limited to project scope.

Fly.io WireGuard Networking

Fly.io uses WireGuard for private networking, providing encrypted communication between your machines globally. You can connect your local development environment to your Fly network securely. The flexibility is greater but requires more networking knowledge.

Static vs Dedicated IPs

This one changed recently and the distinction matters if you're allowlisting. Railway shipped Static Outbound IPs for Pro plans in July 2026, so you can finally give MongoDB Atlas or a partner's firewall a set of addresses to trust. Read the caveat in Railway's docs before you rely on it: "There is no guarantee that the IPv4 addresses assigned to your service are dedicated. They may be shared with other customers." They're also outbound only, and they change if you move regions.

Shared means an allowlist entry for your service is potentially an allowlist entry for someone else's. For an internal firewall rule that's usually acceptable. For a compliance control where the auditor asked "who can reach this database," it's a weaker answer than it looks. Fly's $2/month dedicated IPv4 is genuinely yours and works for inbound traffic too.

Pricing Comparison

The pricing surprise in 2026 is that these two platforms swapped positions. Fly.io used to be the one with a free tier. It isn't anymore.

Railway's model is a plan fee that includes matching credits, then metered usage on top. Fly's is pure usage with no floor, which sounds cheaper and often is for a machine that scales to zero. It's harder to forecast, though, because there's no plan number to anchor on. If you want a bill you can predict before you get it, Railway's is the easier one to reason about.

Best Practices

Generic advice applies to both (private networking for internal traffic, TLS everywhere, review team access on a schedule). The platform-specific parts are what people miss:

On Railway:

• Audit which variables the build genuinely needs. Everything else is unnecessary exposure to your dependency tree.
• Seal anything you never need to read back. It won't keep the value out of the build, but it removes the dashboard and the API as ways to retrieve it.
• Scope shared variables per service instead of linking them everywhere. A service that doesn't reference a variable never receives it.
• Check what your PR environments inherit. A preview deploy running against production credentials is a preview deploy an attacker would love to find.

On Fly.io:

• Use fly secrets for anything sensitive and leave build secrets alone unless you truly need a value during docker build.
• Remember that fly secrets list shows digests, not values. If you didn't record a secret somewhere safe when you set it, you're rotating it, not recovering it.
• Firecracker gives you a kernel boundary between tenants. It gives you nothing between your own app and your own database if the credentials leak.